ASE Automobile (A1–A8) — All Questions

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.

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
• b.a vacuum gauge
• c.a cooling-system pressure tester on the radiator 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 cylinder
• b.an excessively rich air-fuel mixture✓
• c.oil being drawn past the rings
• d.normal water-vapor condensation 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
• b.knock and pinging only on cold starts
• c.excessive blue exhaust smoke that appears 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 form oxides of nitrogen
• c.a lean mixture that burns cleanly
• d.completely normal operation

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 set on the opposite cylinder bank
• c.a misfire and rough running on that cylinder✓
• d.a rich mixture on all cylinders

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 dropping its signal
• b.a loose, damaged, or missing fuel cap✓
• c.low fuel pressure at the injector rail
• d.a failed catalytic converter

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.

A battery's cold cranking amps (CCA) rating specifies the current it can deliver:

• a.at 80 degrees Fahrenheit for a full 60 minutes of continuous discharge
• b.for exactly 15 seconds at normal room temperature under any load
• c.at 0 degrees Fahrenheit for 30 seconds while staying above 7.2 volts✓
• d.only after the charging system has brought it up to a full charge

CCA measures how many amps a fully charged battery can supply at 0 F for 30 seconds while staying at or above 7.2 volts, reflecting cold-start ability. A battery must meet or exceed the vehicle's CCA requirement. Cold weather reduces available cranking power.

A battery's reserve capacity rating indicates how long it can:

• a.accept a charge from the alternator when it is deeply discharged
• b.be stored on a shelf before it will need to be recharged again
• c.supply a set current to run essential loads if the charging system fails✓
• d.deliver its full cold cranking amps rating in freezing weather

Reserve capacity is the minutes a fully charged battery can supply about 25 amps at 80 F while holding above 10.5 volts, showing how long it could run the vehicle with a dead charging system. It complements the CCA rating. A higher number means more backup run time.

A conventional battery capacity (load) test is typically performed by loading the battery to about:

• a.one-half its CCA rating for 15 seconds while watching the voltage✓
• b.the alternator's maximum rated output current for five minutes
• c.its full rated reserve capacity continuously for one full hour
• d.twice its CCA rating for 30 seconds while the engine is cranking

A load test draws roughly half the CCA for 15 seconds; a good battery stays above about 9.6 volts at 70 F. The battery must be adequately charged first for a valid result. Electronic conductance testers estimate capacity without a heavy load.

On a battery with removable caps, a hydrometer measures state of charge by reading the:

• a.internal resistance of the lead plates in each cell
• b.specific gravity of the electrolyte in each cell✓
• c.temperature of the electrolyte
• d.open-circuit voltage at the terminals

A hydrometer measures electrolyte specific gravity, which rises with charge; about 1.265 indicates full charge in a healthy cell. A large variation between cells indicates a defective battery. Readings are temperature-corrected for accuracy.

An absorbed glass mat (AGM) battery differs from a conventional flooded battery in that it:

• a.contains no lead plates whatsoever in its internal construction
• b.is self-charging internally and needs no charging system at all
• c.must always be installed in a completely upside-down position
• d.holds its electrolyte in glass mats, resisting spills and vibration✓

In an AGM battery the electrolyte is absorbed into fiberglass mats between the plates, making it spill-resistant and more vibration-tolerant. AGM batteries often require a charger setting matched to their chemistry. Using the wrong high charging voltage can shorten their life.