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, with excess air scrubbing the insulator clean
• b.cooler than the others because coolant leaked into it
• c.rich, with more fuel than can be fully burned✓
• d.exactly at the stoichiometric ratio

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 that raise effective compression
• b.a spark plug gap set slightly wider than specification✓
• c.excessive ignition timing advanced too far for the fuel
• d.fuel with an octane rating below 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
• b.the frequency at which the crank sensor triggers the module
• c.the turns ratio between its primary and secondary windings✓
• d.the size of the spark plug electrode gap

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 carrying excess fuel to the tank
• b.length of time (pulse width) the PCM holds the injector open✓
• c.spring pressure inside the fuel pressure regulator
• d.resistance printed on the injector's 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 applies only to diesel engines
• c.1 part air to 14.7 parts fuel by weight
• d.about 9.0 parts air to 1 part fuel

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 air flowing past a heated sensing element in the intake
• c.the rotational position of the camshaft
• d.the oxygen left in the exhaust

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 so fewer emissions form
• c.storing unburned fuel for 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
• c.a restricted exhaust that builds backpressure
• d.a leaking injector riching the mixture 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 before the inspection
• c.ignored entirely by the inspection
• d.forced into a 'not ready' state on purpose with a scan tool

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
• b.dieseling or run-on that continues after the key is shut off
• c.knock and pinging 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 speed increases
• b.hard starting only in very cold weather
• c.loss of power that worsens at higher engine speeds✓
• d.a steady misfire at idle that 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 running constantly at high speed
• c.reduced ignition coil dwell time
• d.a lean mixture with excess air causing a 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
• c.coolant leaking past the head gasket
• d.ignition firing the 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 drawn into and burned in the chamber
• c.low compression on one cylinder
• 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.