ASE Automobile (A1–A8) — All Questions

An oxygen (O2) sensor in the exhaust provides feedback to the PCM about:

• a.Ignition timing advance
• b.Engine oil level
• c.The air-fuel ratio (rich or lean)✓
• d.Coolant temperature

The oxygen sensor measures residual oxygen in the exhaust, telling the PCM whether the mixture is rich or lean so it can adjust fuel trim. A lazy or failed sensor degrades fuel control and emissions. Wide-band sensors report the actual ratio rather than just rich/lean.

A diagnostic trouble code (DTC) of P0301 indicates:

• a.A lean condition on bank 1
• b.An EVAP leak
• c.A faulty coolant sensor
• d.A misfire detected in cylinder number 1✓

P0301 is a cylinder-specific misfire code, with the last digit identifying cylinder 1. P0300 indicates a random/multiple-cylinder misfire. Causes include ignition, fuel, or mechanical (compression) problems in that cylinder.

Technician A says a vacuum leak causes a rich condition. Technician B says a vacuum leak adds unmetered air and causes a lean condition, especially at idle. Who is correct?

• a.Both A and B
• b.Technician B only✓
• c.Technician A only
• d.Neither A nor B

A vacuum leak lets extra air enter after the mass airflow sensor, leaning the mixture and often causing a rough idle and positive fuel trim. Technician B is correct. Smoke testing helps locate intake leaks.

Technician A says a positive long-term fuel trim of +20 percent means the PCM is adding fuel to correct a lean condition. Technician B says positive trim means the engine is running rich. Who is correct?

• a.Both A and B
• b.Technician A only✓
• c.Technician B only
• d.Neither A nor B

Positive fuel trim means the computer is adding fuel to correct a lean condition from a vacuum leak, low fuel pressure, or a dirty MAF. Technician B is wrong because rich conditions produce negative trims. Comparing short- and long-term trims helps pinpoint the fault.

Technician A says a rich air-fuel mixture can raise tailpipe CO and HC. Technician B says the oxygen sensor helps the PCM keep the mixture near stoichiometric. Who is correct?

• a.Both A and B✓
• b.Technician A only
• c.Neither A nor B
• d.Technician B only

A rich mixture leaves excess fuel that raises carbon monoxide and hydrocarbons, and the oxygen sensor feedback lets the PCM hold the mixture near the ideal ratio. Both technicians are correct. A lazy sensor degrades fuel control and emissions.

Technician A says the MAF sensor measures the mass of air entering the engine so the PCM can meter fuel. Technician B says a dirty MAF can cause hesitation and fuel-trim errors. Who is correct?

• a.Both A and B✓
• b.Neither A nor B
• c.Technician B only
• d.Technician A only

The MAF sensor measures intake air mass so the PCM meters the correct fuel quantity, and a dirty or faulty MAF causes incorrect fueling, hesitation, and trim errors. Both technicians are correct. Diagnosis includes cleaning or replacing it and checking for unmetered air leaks.

A P0420 code (catalyst efficiency below threshold) most often indicates:

• a.A slipping transmission
• b.A faulty ABS sensor
• c.A degraded catalytic converter or an O2 sensor issue✓
• d.A worn wheel bearing

P0420 sets when the downstream O2 sensor signal mirrors the upstream too closely, suggesting the converter is no longer storing oxygen effectively. Causes include a worn converter, exhaust leaks, or faulty sensors. Underlying misfires or rich running can damage the converter and must be addressed first.

In a modern coil-on-plug ignition system, each coil:

• a.Stores fuel for injection
• b.Is triggered by a mechanical distributor
• c.Fires the spark plug directly on its cylinder, controlled by the PCM✓
• d.Powers all cylinders at once

Coil-on-plug systems place an individual coil at each spark plug, fired by the PCM through the ignition driver, eliminating plug wires and the distributor. This improves spark control and misfire diagnosis. A failed coil typically causes a cylinder-specific misfire code.

A knock sensor allows the PCM to:

• a.Read vehicle speed
• b.Control the fuel pump
• c.Measure exhaust oxygen
• d.Detect detonation and retard ignition timing to protect the engine✓

The knock sensor detects the vibration of detonation so the PCM can retard timing and prevent engine damage. Persistent knock can come from low-octane fuel, carbon buildup, or overheating. A faulty sensor may cause reduced performance or a code.

The EVAP system is designed to:

• a.Increase fuel pressure
• b.Cool the intake charge
• c.Filter engine oil
• d.Capture and burn fuel vapors from the tank rather than venting them✓

The evaporative emission system stores fuel vapors in a charcoal canister and later purges them into the intake to be burned, preventing release to the atmosphere. A loose or faulty gas cap is a common cause of small-leak codes. It is tested for leaks during OBD-II monitors.

A loose or missing fuel cap most commonly sets which type of code?

• a.A coolant temperature code
• b.An ABS code
• c.An EVAP system leak code (such as P0455 or P0457)✓
• d.A misfire code

A poorly sealing fuel cap lets vapors escape and triggers an EVAP leak code, often the first thing to check for such faults. Tightening or replacing the cap and clearing the code frequently resolves it. Smoke testing finds larger leaks in the system.

Technician A says low fuel pressure can cause a lean misfire and hesitation. Technician B says a clogged fuel filter or weak pump can lower fuel pressure. Who is correct?

• a.Both A and B✓
• b.Technician A only
• c.Neither A nor B
• d.Technician B only

Insufficient fuel pressure leans the mixture, causing misfire, hesitation, and lack of power, and a restricted filter or failing pump is a common cause. Both technicians are correct. A fuel-pressure test and volume check confirm delivery problems.

The throttle position sensor (TPS) informs the PCM about:

• a.Battery voltage
• b.Coolant level
• c.Exhaust temperature
• d.Throttle plate angle (driver demand)✓

The TPS reports how far the throttle is open so the PCM can adjust fuel, timing, and transmission strategy. A faulty or dead-spot TPS causes hesitation, surging, or erratic shifts. Its signal is checked with a scan tool or by watching voltage sweep.

A vehicle fails an emissions test for high hydrocarbons (HC). This is most commonly associated with:

• a.A worn wheel bearing
• b.Excessive coolant
• c.Low tire pressure
• d.Unburned fuel from misfire, ignition, or rich-condition problems✓

High HC indicates unburned fuel escaping into the exhaust, typically from misfires, ignition faults, or an overly rich mixture. Diagnosing the misfire or fuel-control fault reduces HC. A failing catalytic converter can also raise tailpipe HC.

The PCM uses the crankshaft position sensor primarily to:

• a.Measure intake air temperature
• b.Detect wheel slip
• c.Read exhaust oxygen
• d.Determine engine speed and piston position for ignition and fuel timing✓

The crankshaft position sensor tells the PCM engine rpm and crank position so it can time spark and injection. Loss of this signal usually prevents the engine from starting. A cam/crank correlation code indicates timing has slipped between the two.

On an OBD-II vehicle, the data link connector (DLC) used to retrieve codes is standardized as a:

• a.9-pin round plug under the hood
• b.4-pin trailer connector
• c.16-pin connector, usually under the dash✓
• d.2-pin connector at the battery

OBD-II mandates a standardized 16-pin DLC, typically located under the driver's side dash, for scan-tool access. This standard allows generic tools to read codes and data across manufacturers. Certain pins carry power, ground, and communication signals.

The exhaust gas recirculation (EGR) system reduces which emission by lowering combustion temperature?

• a.Oxides of nitrogen (NOx)✓
• b.Carbon monoxide (CO)
• c.Carbon dioxide (CO2)
• d.Hydrocarbons (HC)

EGR routes inert exhaust gas back into the intake to lower peak combustion temperature, reducing the formation of NOx. A clogged EGR passage can cause knock and high NOx, while a stuck-open valve causes rough idle. Its operation is monitored by OBD-II.

A no-start with no spark and no injector pulse, but a good battery and cranking, most likely points to:

• a.A worn water pump
• b.A leaking A/C condenser
• c.A failed crankshaft position sensor or PCM/power issue✓
• d.A clogged cabin filter

When both spark and injector pulse are absent during cranking, a shared input such as the crankshaft position sensor or a PCM power/ground problem is a prime suspect. Scan-tool rpm during cranking helps confirm the crank signal. Systematic checks of power, ground, and reference sensors isolate the cause.

A spark plug removed from one cylinder is dry and coated with dry, black soot, while the others are tan. This most directly indicates that cylinder is running:

• a.lean, because the excess air has scrubbed the insulator clean of any color
• b.cooler than the others because coolant has leaked into that cylinder
• c.rich, with more fuel than can be fully burned✓
• d.exactly at the ideal stoichiometric ratio for that operating condition

Dry, black, sooty carbon on the insulator points to an over-rich mixture in that cylinder, such as a leaking injector or a fault pulling fuel trim positive there. A tan or light-gray insulator is normal. Wet black deposits instead suggest oil, and white blistering suggests overheating.

The heat range of a spark plug refers to how quickly the plug:

• a.can produce a spark once the ignition module has energized the coil
• b.raises the temperature of the incoming air-fuel charge before ignition
• c.develops secondary voltage across its electrode gap under heavy load
• d.transfers combustion heat away from its firing tip to the cylinder head✓

Heat range describes the plug's ability to conduct heat from the tip into the head. A colder plug sheds heat faster and resists pre-ignition in hot engines, while a hotter plug runs the tip warmer to burn off deposits. Using the specified heat range prevents fouling or detonation.

Spark knock (detonation) heard as a metallic pinging under acceleration can be caused by all of the following EXCEPT:

• a.carbon deposits in the chamber that raise the engine's effective compression
• b.a spark plug gap set slightly wider than specification✓
• c.excessive ignition timing that is advanced too far for the fuel being burned
• d.fuel with an octane rating that is lower than what the engine requires

Detonation is uncontrolled combustion after the spark, driven by too much timing, low octane, carbon buildup, overheating, or a lean mixture. A slightly wide plug gap affects spark, not the pressure-and-temperature conditions that cause knock. The knock sensor retards timing to protect the engine.

An ignition coil steps 12 volts up to the tens of thousands of volts needed to fire the plug primarily because of:

• a.the resistance built into the spark plug wire on the secondary output side
• b.the frequency at which the crankshaft position sensor triggers the module
• c.the turns ratio between its primary and secondary windings✓
• d.the size of the air gap designed between the spark plug electrodes

A coil is a transformer: the large turns ratio between the many secondary windings and the few primary windings multiplies the induced voltage when the primary field collapses. This produces the high secondary voltage that jumps the plug gap. Weak primary current lowers available secondary voltage.

In a port fuel-injection system, the quantity of fuel delivered by each injector is controlled mainly by the:

• a.diameter of the fuel return line that carries excess fuel back to the tank
• b.length of time (pulse width) the PCM holds the injector open✓
• c.spring pressure that is set inside the mechanical fuel pressure regulator
• d.resistance value that is printed on the injector's electrical connector

With fuel pressure held relatively constant, the PCM meters fuel by varying injector on-time, called pulse width. Longer pulse width delivers more fuel for acceleration or enrichment. Fuel trims adjust pulse width to keep the mixture near stoichiometric.

Technician A says a clogged or restricted fuel injector can cause a lean misfire on that cylinder. Technician B says a leaking injector can cause a rich condition and hard starting. Who is correct?

• a.Both A and B✓
• b.Technician B only
• c.Technician A only
• d.Neither A nor B

A restricted injector starves its cylinder and causes a lean misfire, while a leaking injector adds extra fuel that riches the mixture and can flood a cylinder on start-up. Both technicians are correct. An injector balance test compares delivery among cylinders.

The theoretically ideal (stoichiometric) air-fuel ratio for a gasoline engine is approximately:

• a.14.7 parts air to 1 part fuel by weight✓
• b.stoichiometry is a diesel-only concept not used on gasoline engines
• c.1 part air to 14.7 parts fuel by weight, the reverse proportion
• d.about 9.0 parts air to 1 part fuel, which is a very rich mixture

Stoichiometric combustion of gasoline occurs near 14.7:1 air to fuel by mass, the ratio at which the three-way catalyst works most efficiently. The PCM uses oxygen-sensor feedback to hold the average close to this value. Richer or leaner operation is used only for specific conditions.

When the engine is cold at start-up, the PCM operates in open loop, meaning it:

• a.shuts off all of the fuel injectors completely until the engine warms up
• b.doubles the ignition timing advance in order to speed up the warm-up
• c.relies only on the downstream oxygen sensor to control the fuel mixture
• d.ignores the oxygen sensor and uses programmed values to set fuel delivery✓

In open loop the oxygen sensors are not yet at operating temperature, so the PCM meters fuel from stored maps based on coolant temperature, load, and rpm. Once the sensors are hot and the engine is warm, it enters closed loop and trims fuel from sensor feedback.

Short-term fuel trim (STFT) differs from long-term fuel trim (LTFT) in that STFT:

• a.controls ignition timing advance and retard rather than fuel delivery
• b.makes rapid, moment-to-moment corrections from oxygen-sensor feedback✓
• c.is calculated only from the mass airflow reading with no feedback loop
• d.is a permanent stored value that can only be cleared with a scan tool

Short-term fuel trim reacts instantly to oxygen-sensor switching, while long-term fuel trim is a learned, slower average that offsets persistent deviations. Reading both together helps distinguish momentary from ongoing mixture problems. Large positive trims indicate the PCM is adding fuel to correct lean operation.

A manifold absolute pressure (MAP) sensor provides the PCM with information about:

• a.engine load, based on intake manifold pressure and vacuum✓
• b.the mass of the air flowing past a heated sensing element in the intake
• c.the exact rotational position of the camshaft during each engine cycle
• d.the amount of oxygen left in the exhaust after the catalytic converter

A MAP sensor reports manifold pressure, which reflects engine load in a speed-density fuel strategy. High vacuum (low pressure) indicates light load, while low vacuum (near atmospheric) indicates heavy load. A faulty MAP signal skews fuel and timing calculations.

An intake air temperature (IAT) sensor is used by the PCM to:

• a.measure the temperature of the engine coolant as it leaves the head
• b.correct fuel and timing for the density of the incoming air✓
• c.regulate the fuel pressure that is maintained inside the injector rail
• d.count the engine's revolutions in order to drive the tachometer

Because cold air is denser and carries more oxygen than warm air, the IAT reading lets the PCM fine-tune fuel delivery and spark for air density. A sensor reading falsely warm can lean the mixture. It works together with the MAF or MAP input.

A wide-band (air-fuel ratio) oxygen sensor has an advantage over a conventional narrow-band sensor because it:

• a.can measure how rich or lean the mixture is across a wide range✓
• b.reports exhaust gas temperature to the PCM instead of oxygen content
• c.reaches full operating temperature without needing any heater circuit
• d.produces a usable signal only when the mixture is exactly 14.7 to 1

A narrow-band sensor mainly switches high or low around stoichiometric, while a wide-band sensor reports the actual air-fuel ratio over a broad span. This gives the PCM finer fuel control and faster, more precise diagnostics. Both types still require heaters for quick activation.

The downstream (post-catalyst) oxygen sensor is used mainly to:

• a.measure the temperature of the exhaust stream ahead of the catalyst
• b.command and control the engine's idle speed under the PCM's direction
• c.monitor how well the catalytic converter stores and releases oxygen✓
• d.provide the primary fuel-trim feedback the PCM uses for the injectors

The rear oxygen sensor watches the exhaust after the catalyst; a healthy converter smooths out the oxygen swings so the rear signal stays fairly steady. When the rear sensor begins to mirror the front, catalyst efficiency has dropped and a P0420-type code may set. The front sensor handles fuel trim.

A three-way catalytic converter reduces tailpipe emissions by:

• a.trapping soot particles from the exhaust the way a diesel filter does
• b.cooling the exhaust gases so fewer emissions can form farther downstream
• c.storing unburned fuel from the exhaust for reuse on the next cold start
• d.oxidizing HC and CO while reducing NOx into harmless gases✓

The three-way catalyst oxidizes hydrocarbons and carbon monoxide into water and carbon dioxide and reduces oxides of nitrogen into nitrogen and oxygen. It works best when the mixture is held near stoichiometric. Misfires, oil, or coolant contamination can overheat and destroy it.

Technician A says a persistent engine misfire can overheat and damage the catalytic converter. Technician B says a flashing malfunction indicator lamp (MIL) warns of a misfire severe enough to harm the catalyst. Who is correct?

• a.Neither A nor B
• b.Technician A only
• c.Both A and B✓
• d.Technician B only

Raw fuel from a misfire ignites in the catalyst and drives its temperature high enough to cause damage, and the PCM flashes the MIL to warn of that catalyst-damaging misfire. Both technicians are correct. The vehicle should be driven gently or stopped until the misfire is repaired.

A P0171 code (system too lean, bank 1) could be caused by any of the following EXCEPT:

• a.a contaminated mass airflow sensor that under-reports the incoming air
• b.low fuel pressure caused by a weak fuel pump or a restricted filter
• c.a leaking fuel injector adding extra fuel to the cylinders✓
• d.an unmetered vacuum leak downstream of the mass airflow sensor

A lean code results from too much air or too little fuel: vacuum leaks, low fuel pressure, or an under-reporting MAF all lean the mixture. A leaking injector adds fuel and would cause a rich condition instead. Checking fuel trims and fuel pressure narrows the cause.

A P0300 (random/multiple-cylinder misfire) code, as opposed to a single-cylinder code, more often points to a problem that is:

• a.common to several cylinders, such as fuel pressure or a vacuum leak✓
• b.isolated to a single coil-on-plug ignition coil on one cylinder
• c.caused only by one leaking or restricted fuel injector
• d.limited to one fouled or worn spark plug on a single cylinder

Because P0300 reports misfires spread across cylinders, the cause is usually something they share: low fuel pressure, a large vacuum leak, EGR flooding the intake, or a weak ignition supply. A single-cylinder code like P0301 points to that cylinder's plug, coil, or injector. Fuel-trim and freeze-frame data guide the diagnosis.

The EVAP purge valve (canister purge solenoid) controls:

• a.the fuel pressure that is maintained in the rail during acceleration
• b.the recirculation of exhaust gas back into the intake manifold for NOx
• c.the flow of stored fuel vapor from the charcoal canister into the intake✓
• d.the amount of fresh air that enters the engine through the throttle body

The purge valve meters fuel vapor from the EVAP canister into the intake to be burned when conditions allow. A valve stuck open acts like a vacuum leak and can cause rough idle or stalling. A stuck-closed valve prevents purging and can set an EVAP flow code.

Fuel trims that read strongly positive at idle but return close to zero at higher rpm most likely indicate:

• a.a vacuum leak whose effect is diluted as airflow increases✓
• b.a mass airflow sensor that over-reports the incoming airflow at idle
• c.a restricted exhaust system that builds up backpressure at higher speed
• d.a leaking fuel injector that keeps riching the mixture only at idle

A fixed-size vacuum leak adds a large percentage of unmetered air at idle when total airflow is low, driving fuel trim positive; at higher rpm that same leak is a small fraction of total flow, so trim normalizes. This rpm-dependent pattern is a classic vacuum-leak fingerprint. Smoke testing helps locate it.

Before an OBD-II vehicle can pass a state emissions inspection, its readiness monitors generally must be:

• a.run to completion, showing 'ready' or 'complete' status✓
• b.cleared to zero by disconnecting the battery just before the inspection
• c.ignored entirely, because the monitors have no bearing on the inspection
• d.forced into a 'not ready' state on purpose using a scan tool command

Readiness monitors are self-tests the PCM runs during specific drive conditions; most programs require them complete so the system can confirm emissions components work. Clearing codes or disconnecting the battery resets monitors to 'not ready,' which can fail an inspection until a drive cycle is completed. A specific drive cycle sets them.

The idle air control (IAC) valve on a cable-throttle engine is used to:

• a.adjust the ignition timing advance during closed-throttle idle operation
• b.meter fuel directly into the intake ports whenever the engine idles
• c.open a passage that recirculates exhaust gas into the intake at idle
• d.regulate airflow bypassing the throttle plate to control idle speed✓

The IAC valve opens or closes a passage that lets air bypass the closed throttle plate, letting the PCM raise idle for cold start or accessory loads and lower it when warm. A dirty or sticking IAC causes unstable or stalling idle. Electronic-throttle engines control idle by moving the throttle plate itself.

On an electronic throttle control (drive-by-wire) system, the PCM verifies driver intent using:

• a.the vehicle speed sensor signal, from which it infers pedal demand
• b.two accelerator-pedal position sensors that must agree with each other✓
• c.only the mass airflow sensor reading with no separate pedal confirmation
• d.a mechanical cable linked directly from the pedal to the throttle plate

Drive-by-wire uses redundant accelerator-pedal position sensors and redundant throttle-position sensors so the PCM can cross-check the signals for safety. If the sensors disagree, the system enters a reduced-power or limp mode. This redundancy replaces the old mechanical throttle cable.

Freeze-frame data stored by the PCM is valuable because it records:

• a.a continuous real-time video recording of all the sensor waveforms
• b.the operating conditions present at the moment a fault was detected✓
• c.a running history of every trouble code the vehicle has ever set
• d.the vehicle owner's complete maintenance and repair service records

Freeze frame captures a snapshot of parameters such as rpm, load, coolant temperature, and fuel trim when the code set, helping recreate the fault conditions. Reviewing it guides the technician toward when and why the problem occurs. It is especially useful for intermittent faults.

Ignition timing that is excessively retarded from specification will most likely cause:

• a.an abnormally high idle speed even with the throttle held fully closed
• b.dieseling or run-on that continues after the ignition key is shut off
• c.engine knock and pinging that occurs mainly under a light-throttle load
• d.reduced power, poor fuel economy, and possible overheating✓

Retarded timing fires the mixture too late, wasting energy as heat in the exhaust, which lowers power and economy and can overheat the engine. Over-advanced timing, by contrast, tends to cause knock. Modern engines set base timing by the PCM using the crank sensor.

Technician A says an EGR valve stuck open can cause a rough idle or stalling. Technician B says an EGR valve stuck closed can allow combustion temperatures to rise and increase NOx. Who is correct?

• a.Neither A nor B
• b.Technician B only
• c.Both A and B✓
• d.Technician A only

Exhaust gas recirculation dilutes the intake charge to lower peak combustion temperature; stuck open at idle it acts like a vacuum leak and causes rough idle or stalling, while stuck closed it lets temperatures and NOx climb. Both technicians are correct. EGR faults commonly set flow-related DTCs.

A restricted exhaust system, such as a plugged catalytic converter, typically produces which driveability symptom?

• a.a rough idle at low rpm that smooths out as engine speed increases
• b.hard starting that appears only during very cold outside weather
• c.loss of power that worsens at higher engine speeds✓
• d.a steady misfire at idle that completely disappears under load

A clogged converter or crushed pipe raises backpressure, choking the engine most at high rpm when it needs to move the most exhaust, so power fades as speed climbs. A backpressure or vacuum test helps confirm restriction. Overheating from a rich condition or misfire is a common cause of a melted converter.

An engine coolant temperature (ECT) sensor that reads colder than the engine actually is will most likely cause:

• a.an overly rich mixture and poor fuel economy✓
• b.the electric cooling fan to run constantly at high speed while cold
• c.reduced ignition coil dwell time and a correspondingly weak spark
• d.a lean mixture with excess air causing a lean misfire at idle

The PCM enriches the mixture when it believes the engine is cold, so a sensor stuck reading low keeps the engine in warm-up enrichment, wasting fuel and fouling plugs. It may also delay closed-loop operation. A sensor reading falsely hot has the opposite effect.

Excessive carbon monoxide (CO) in the exhaust of a gasoline engine most directly indicates:

• a.a rich air-fuel mixture with incomplete combustion✓
• b.a very lean air-fuel mixture that leaves excess oxygen in the exhaust
• c.coolant that is leaking past the head gasket into the combustion chamber
• d.an ignition system that is firing the spark plugs far too early

High CO is a product of incomplete combustion from too much fuel, so it is a rich-mixture indicator. High hydrocarbons point to misfire or unburned fuel, while high NOx points to high combustion temperature. Gas-analyzer readings help pinpoint the fault.

High oxides of nitrogen (NOx) in the exhaust are most closely associated with:

• a.a very rich air-fuel mixture with too much fuel at idle
• b.excess engine oil being drawn into and burned in the chamber
• c.low engine compression on one cylinder from a burned valve
• d.abnormally high combustion temperatures✓

NOx forms when combustion temperatures climb high enough to combine nitrogen and oxygen, so causes include a lean mixture, a non-functioning EGR system, or engine overheating. Recirculating exhaust lowers peak temperature and NOx. A cooling-system or EGR fault is a common contributor.

The camshaft position (CMP) sensor signal is used by the PCM primarily to:

• a.control the position of the electronic drive-by-wire throttle plate
• b.measure the mass of the air that is entering the intake manifold
• c.identify which cylinder is on compression for sequential injection✓
• d.monitor the oxygen-storage efficiency of the catalytic converter

The camshaft sensor tells the PCM cylinder identification and cam timing so it can fire injectors sequentially and time the coils correctly. Paired with the crankshaft sensor, it also detects cam-to-crank correlation problems from timing-chain wear. A lost CMP signal can cause a no-start or extended crank.

A variable valve timing (VVT) system uses an oil control solenoid to:

• a.vary camshaft timing by directing oil pressure to a cam phaser✓
• b.change the cylinder firing order under heavy engine load conditions
• c.regulate the fuel-rail pressure delivered during hard acceleration
• d.open a bypass passage around the throttle plate during warm idle

VVT advances or retards the camshaft by using PCM-controlled oil pressure through a solenoid to move a cam phaser. Low or dirty oil, or a stuck solenoid, can set VVT performance codes and hurt driveability. Correct oil level and viscosity are important for proper operation.

Technician A says a PCV valve stuck open can act like a vacuum leak and lean the mixture. Technician B says a plugged PCV system can cause oil leaks and sludge. Who is correct?

• a.Neither A nor B
• b.Technician B only
• c.Technician A only
• d.Both A and B✓

A PCV valve stuck open lets extra unmetered air into the intake, which the PCM sees as a lean condition, while a plugged system traps blow-by pressure that pushes past seals and promotes sludge. Both technicians are correct. The PCV system should be checked when diagnosing lean codes or oil leaks.

A flooded engine that cranks but will not start after repeated attempts can often be cleared by:

• a.cranking with the throttle held fully closed while cycling the key on and off
• b.pumping the throttle pedal rapidly several times just before cranking
• c.spraying a little water into the throttle body to wash the excess fuel out
• d.holding the throttle wide open while cranking to enter clear-flood mode✓

Most PCMs enter a clear-flood mode that cuts injector pulse width when the throttle is held wide open during cranking, letting excess fuel clear. Pumping a fuel-injected throttle does not add fuel and can worsen flooding. Once it starts, the mixture returns to normal control.

A technician suspects no spark on a coil-on-plug engine. The best tool to confirm the presence of spark is:

• a.a fuel pressure gauge teed into the engine's fuel supply rail
• b.a vacuum gauge connected to a port on the intake manifold
• c.a cooling-system pressure tester attached to the radiator filler neck
• d.a spark tester connected in place of the plug✓

A spark tester provides a calibrated gap to verify that the coil produces a strong spark under compression-like load. Simply pulling a plug wire can give a misleading result and risks a shock. Once spark is confirmed or ruled out, the technician checks the coil, driver, and triggering sensors.

A knock (detonation) sensor is best described as a device that:

• a.counts the crankshaft revolutions used to drive the dashboard tachometer
• b.detects engine vibration frequencies caused by abnormal combustion✓
• c.reports intake manifold vacuum and engine load conditions to the PCM
• d.measures the temperature of the exhaust manifold near the exhaust port

The knock sensor is a piezoelectric device tuned to the vibration frequency of detonation; when it senses knock, the PCM retards timing to protect the engine. A faulty sensor or wiring can cause reduced power from unnecessary timing retard or unheard knock. Torque specification matters when installing it.

Black smoke from the tailpipe of a gasoline engine most directly indicates:

• a.coolant leaking past a head gasket into the combustion chamber
• b.an excessively rich air-fuel mixture✓
• c.engine oil being drawn past the rings and burned in the cylinders
• d.normal water-vapor condensation seen briefly on a cold morning start

Black smoke is unburned fuel from a rich mixture, caused by faults such as leaking injectors, high fuel pressure, or a false-cold coolant sensor. White smoke suggests coolant, and blue suggests oil. A gas analyzer confirms high CO and HC with a rich condition.

A vacuum leak at the intake manifold typically causes an idle that is:

• a.unaffected because idle is entirely computer-controlled
• b.high or rough because of the extra unmetered air✓
• c.low and smooth because the mixture becomes noticeably richer
• d.normal until the engine reaches full operating temperature

Unmetered air from a vacuum leak leans the idle mixture and can raise or destabilize idle speed, and fuel trims go positive to compensate. Smoke testing or spraying a small amount of throttle-body cleaner near suspect areas helps locate it. Idle strategy tries to compensate but often cannot fully mask the leak.

Low fuel pressure caused by a weak pump or clogged filter most commonly produces which driveability complaint?

• a.a high, surging idle speed that will not settle down to normal
• b.engine knock and pinging that occurs only during cold engine starts
• c.excessive blue exhaust smoke that appears mainly on deceleration
• d.hesitation, lack of power, and lean misfire under load✓

When demand is high, low fuel pressure cannot supply enough fuel, so the engine hesitates, lacks power, and may lean-misfire under load. A fuel pressure gauge and volume test confirm supply. A restricted filter or failing pump are common causes.

The crankshaft position sensor and camshaft position sensor together allow the PCM to detect:

• a.the temperature of the intake air charge entering the engine
• b.the amount of fuel remaining in the vehicle's fuel tank
• c.the oxygen-storage efficiency of the catalytic converter
• d.timing-chain wear through a cam-to-crank correlation error✓

By comparing crankshaft and camshaft signals, the PCM verifies valve timing; a stretched timing chain shifts the relationship and sets a correlation code. This can cause rough running and reduced power. Verifying mechanical timing may be required after a correlation DTC.

An engine that starts and runs briefly, then stalls, and will restart only after sitting, may be exhibiting a classic symptom of:

• a.a single fouled spark plug that is causing one steady misfire
• b.a slightly loose gas cap that has set a small EVAP system code
• c.a failing crankshaft position sensor that drops out when hot✓
• d.a worn serpentine drive belt that is slipping under electrical load

A crank sensor that fails when it heats up can cut the signal, stalling the engine until it cools enough to work again. Testing while heating the sensor or watching rpm data during the stall helps confirm it. Heat-related intermittent sensor faults are a common driveability puzzle.

Technician A says a mass airflow sensor that under-reports airflow will make the mixture lean. Technician B says cleaning a contaminated MAF sensor with an approved cleaner can restore accurate readings. Who is correct?

• a.Both A and B✓
• b.Neither A nor B
• c.Technician B only
• d.Technician A only

A MAF that reads low tells the PCM less air is entering than truly is, so it under-fuels and leans the mixture, and dirt on the sensing element is a frequent cause that MAF-specific cleaner can correct. Both technicians are correct. Never touch or scrub the delicate sensing wire.

A gasoline engine that fails an emissions test for high hydrocarbons (HC) is most likely experiencing:

• a.a misfire leaving unburned fuel in the exhaust✓
• b.high combustion temperatures that are forming oxides of nitrogen
• c.a lean air-fuel mixture that leaves excess air but burns cleanly
• d.completely normal operation that is well within all emission limits

Hydrocarbons are unburned fuel, so high HC points to a misfire from ignition, fuel, or compression faults, or a failing catalyst. High CO indicates a rich mixture, while high NOx indicates high temperature. Finding and fixing the misfire lowers HC.

The purpose of the secondary air injection system on some vehicles is to:

• a.recirculate exhaust gas back into the intake to help lower NOx
• b.add extra fuel into the exhaust to raise catalyst temperature at idle
• c.add air to the exhaust so the catalyst warms and oxidizes emissions faster✓
• d.cool the exhaust manifold and downpipe during periods of heavy load

Secondary air injection pumps fresh air into the exhaust during cold start so extra oxygen helps oxidize HC and CO and warms the catalyst quickly. A stuck valve or failed pump can set related codes. It operates only briefly after start-up.

A cylinder with very low compression will most likely cause:

• a.an abnormally high charging-system voltage from the alternator
• b.a false lean trouble code being set on the opposite cylinder bank
• c.a misfire and rough running on that cylinder✓
• d.a rich air-fuel mixture affecting all of the cylinders at once

Low compression means that cylinder cannot make normal power, producing a misfire and rough idle. Causes include burned valves, worn rings, or a blown head gasket, confirmed with a compression or leak-down test. The misfire may set a cylinder-specific code.

Ethanol content in gasoline matters to the fuel system because ethanol:

• a.removes any need for oxygen-sensor feedback in the fuel-control system
• b.cannot be detected or compensated for in any way by the PCM
• c.changes the stoichiometric ratio, so flex-fuel systems adjust fueling✓
• d.raises the fuel pressure inside a sealed tank entirely on its own

Ethanol requires a richer air-fuel ratio than straight gasoline, so flex-fuel vehicles use a fuel composition sensor or fuel-trim learning to add fuel as ethanol content rises. Incorrect fuel can cause lean or rich operation. Fuel-system materials are chosen to resist ethanol.

A P0442 (small EVAP leak) code is frequently caused by:

• a.a worn crankshaft position sensor that intermittently drops its signal
• b.a loose, damaged, or missing fuel cap✓
• c.low fuel pressure at the injector rail during heavy acceleration
• d.a failed catalytic converter substrate that has broken apart inside

A small EVAP leak commonly traces to a fuel cap that is loose, cracked, or has a bad seal, letting vapor escape. The system checks itself by pressurizing or applying vacuum and watching for decay. Tightening or replacing the cap often clears it after a drive cycle.

An injector balance test is used to:

• a.check the oxygen-storage efficiency of the catalytic converter
• b.compare fuel delivery among injectors by measuring pressure drop✓
• c.measure the electrical resistance of the ignition coil primary winding
• d.verify the internal calibration of the mass airflow sensor element

Pulsing each injector for the same time and comparing the fuel-rail pressure drop, or the rpm change from cylinder power balance, reveals a weak or leaking injector. A cylinder that differs sharply signals a fueling problem. This isolates a mechanical injector fault from other misfire causes.

Spark plug gap is important because a gap that is too wide will:

• a.raise the effective compression pressure inside that cylinder
• b.fire the mixture far too easily and rapidly foul the plug with raw fuel
• c.require more voltage to fire and can cause misfires under load✓
• d.lower peak combustion temperature and reduce the formation of NOx

A wider gap needs higher secondary voltage to jump; if the coil cannot supply it under load, the plug misfires. Too small a gap gives a weak spark that may not reliably ignite the mixture. Setting the specified gap ensures consistent ignition.

Technician A says an engine that runs well cold but stumbles once warm may have a coolant temperature sensor or fuel-delivery fault. Technician B says scan-tool live data helps confirm which sensor reading is out of range. Who is correct?

• a.Both A and B✓
• b.Technician A only
• c.Neither A nor B
• d.Technician B only

A warm stumble can come from a sensor whose reading shifts with temperature or from fuel delivery that falls off when hot, and comparing live data to known-good values pinpoints the offending signal. Both technicians are correct. Freeze-frame and fuel-trim data guide the diagnosis.

The oxygen sensor heater circuit is important because it:

• a.brings the sensor to operating temperature so closed loop begins sooner✓
• b.measures the exhaust backpressure present at the sensor
• c.produces the sensor's output voltage signal all by itself
• d.cools the sensor to protect it from exhaust heat damage

A built-in heater lets the oxygen sensor reach its roughly 600-degree operating temperature soon after start-up, so the engine enters closed loop and reduces emissions faster. A failed heater sets a heater-circuit code and delays closed loop. The heater is a separate circuit from the sensing element.

A vehicle sets a P0128 (coolant temperature below thermostat regulating temperature) code most often because:

• a.the radiator pressure cap is holding far too much system pressure
• b.the water pump is circulating the coolant far too quickly to warm it
• c.the electric cooling fan never turns on at any coolant temperature
• d.the thermostat is stuck open and the engine never fully warms✓

P0128 means the engine did not reach expected temperature in the expected time, and a thermostat stuck open is the usual cause, letting coolant flow before warm-up is complete. This wastes fuel and delays full closed-loop efficiency. Verifying warm-up with a scan tool confirms the fault.

Mode 6 data on an OBD-II scan tool is useful because it shows:

• a.a live oscilloscope trace of the current through each ignition coil
• b.the vehicle's complete repair and maintenance service history
• c.the exact street location where the diagnostic code first set
• d.the results and limits of the PCM's non-continuous self-tests✓

Mode 6 reports the pass/fail test values and thresholds behind the readiness monitors, such as catalyst and EVAP tests, so a technician can spot a component trending toward failure before a code sets. Reading it alongside freeze frame aids diagnosis. Test IDs must be matched to the vehicle's definitions.

A dirty or sticking throttle body can cause a driveability complaint of:

• a.a steady high-speed vibration that is felt through the driver's seat
• b.an excessively high charging voltage seen at highway cruising speed
• c.unstable idle, stalling, or hesitation from restricted airflow✓
• d.engine knock that appears only under very heavy acceleration

Carbon buildup on the throttle bore and plate disturbs the small airflow that controls idle, causing rough idle, stalling, or tip-in hesitation. Cleaning the throttle body and, on some vehicles, performing an idle relearn restores normal operation. Electronic throttles may need a relearn after cleaning.

When diagnosing a misfire, performing a cylinder power balance test helps by:

• a.identifying which cylinder contributes least to engine output✓
• b.testing the state of charge and condition of the vehicle's battery
• c.measuring the precise octane rating of the fuel in the tank
• d.checking the front-wheel alignment angles such as camber and toe

A power balance test disables cylinders one at a time (or uses scan-tool misfire counts) to find the weak cylinder by its small effect on rpm. The cylinder that changes engine speed the least is the suspect. Follow-up compression, injector, and ignition tests find the root cause.

A restricted or plugged fuel injector on one cylinder, compared with a leaking injector, is more likely to cause:

• a.an overcharging condition in the vehicle's charging system
• b.black exhaust smoke coming from all of the cylinders under load
• c.a lean misfire and hesitation from that one cylinder✓
• d.a rich condition and hard starting that affects the whole engine

A restricted injector delivers too little fuel to its cylinder, leaning it out and causing a misfire or hesitation, while a leaking injector adds fuel and riches the mixture. Comparing symptoms helps direct the diagnosis. An injector balance or flow test confirms which injector is at fault.