Your FREE ASE A8 Flashcards 2026 – 200+ Cards

How well an engine starts, idles, accelerates, and runs cleanly — and the fuel, ignition, emissions, and computer systems that control it. The focus of ASE A8.

An operating mode (usually a cold engine) in which the ECM ignores the oxygen sensor and fuels from preset values until the system warms up.

The warmed-up mode in which the ECM uses oxygen-sensor feedback to continuously trim fuel and hold the air-fuel ratio near 14.7:1.

The ideal gasoline air-fuel ratio — about 14.7:1 by weight (λ = 1) — at which combustion is most complete and the catalytic converter works best.

Air-fuel ratio relative to stoichiometric: λ = 1 is 14.7:1, λ < 1 is rich, λ > 1 is lean.

The percentage the ECM adjusts fuel from its base calculation. Positive = adding fuel (lean correction); negative = removing fuel (rich correction).

The instant fuel correction the ECM makes in response to the oxygen sensor, swinging up and down moment to moment.

The learned, averaged fuel correction stored over time. A large LTFT signals a persistent rich or lean condition.

The ECM is ADDING fuel, so it sensed a LEAN exhaust. Suspect a vacuum leak, low fuel pressure, a weak pump, or a MAF reading low.

The ECM is SUBTRACTING fuel, so it sensed a RICH exhaust. Suspect high fuel pressure, a leaking injector, a restricted air filter, or a MAF reading high.

Too much air or too little fuel (λ > 1). Causes positive fuel trim, possible misfire, and lean codes (P0171/P0174).

Too much fuel or too little air (λ < 1). Causes negative fuel trim, black smoke, fouled plugs, and rich codes (P0172/P0175).

Air entering after the MAF that the ECM didn't measure. Leans the mixture — worse at idle (high positive trim), better at higher RPM.

A cylinder failing to fire properly — from ignition, fuel, or compression. Causes rough idle, lost power, and a flashing or steady MIL.

An active misfire severe enough to damage the catalytic converter. Stop driving and diagnose the misfire immediately.

A stored emissions-related fault that is not an immediate threat. Retrieve the code and diagnose.

A misfire across multiple cylinders, pointing to a shared cause like a vacuum leak, low fuel pressure, or contaminated fuel.

A misfire isolated to one cylinder, pointing to that cylinder's spark plug, coil, injector, or a compression problem.

Uncontrolled, spontaneous combustion AFTER the spark. Caused by low octane, too much advance, lean/hot mixture, carbon, or high compression.

The mixture igniting BEFORE the spark, from a hot spot such as a glowing carbon deposit or an overheated spark plug.

Coolant entering the combustion chamber — usually a blown head gasket or cracked head. (Thin cold-start vapor is normal.)

Oil burning in the cylinder — worn rings, worn valve guides, or bad valve stem seals.

A rich air-fuel mixture — too much fuel or too little air (clogged air filter, leaking injector, faulty sensor).

A momentary loss of power on acceleration. Often a lean transition, a weak accelerator-pump function, low fuel pressure, or a TPS fault.

A repeated rise and fall in engine speed at steady throttle. Causes include a lean condition, EGR fault, contaminated MAF, or a faulty TPS.

An engine that keeps running after the key is off, from hot spots or carbon deposits igniting the mixture without spark.

Difficult starting when warm. Suspect a leaking injector flooding the engine, a hot-soak fuel-pressure loss, or a coolant-temp sensor fault.

Difficult starting when cold. Suspect low fuel pressure, a faulty coolant-temp sensor (no cold enrichment), weak ignition, or low compression.

Unstable idle. Causes include vacuum leaks, a stuck-open EGR, a misfire, a dirty throttle body, or an IAC/idle-control fault.

Reads manifold vacuum at idle. Steady low = vacuum leak; fluctuating = burned valve; slowly dropping at higher RPM = restricted exhaust.

Disables one cylinder at a time and watches RPM. Little or no RPM drop = that cylinder is weak or dead.

Measures cranking pressure per cylinder to judge how well rings, valves, and the head gasket seal. Used to rule out a mechanical cause of a misfire.

Applies compressed air to a cylinder at TDC and reads the percent leaking; locates the leak (valves, rings, or head gasket).

The highest point of piston travel. Many ignition-timing and diagnostic references use a cylinder at TDC on the compression stroke.

The pressure difference below atmospheric in the intake manifold; a key drivability clue and the power source for many emissions devices.

The engine cranks but doesn't start. Check for spark, fuel, and a crank position signal — a dead CKP sensor is a classic cause.

ASE A8 rewards diagnosing the underlying cause, not just the symptom or the code. Confirm with live data before replacing a part.

Any concern with how the engine runs — stalling, hesitation, misfire, poor economy, or a check-engine light. The starting point of A8 diagnosis.

A gasket or boot leak that lets in unmetered air. Causes a lean condition and a vacuum-leak-like rough idle.

Introducing test smoke into the intake to find vacuum/boot leaks by watching where it escapes — a fast way to confirm an unmetered-air leak.

The signature ASE question format presenting two statements; you decide whether A only, B only, both, or neither is correct.

Short-Term and Long-Term Fuel Trim — the instant and learned fuel corrections; positive = adding fuel (lean), negative = removing fuel (rich).

At wide-open throttle the ECM richens the mixture for power and ignores the O₂ sensor (temporary open loop).

A misfire from too little fuel (low fuel pressure, vacuum leak, lean injector). Raises HC; confirmed by positive fuel trim plus a misfire code.

A misfire from too much fuel (leaking injector, high pressure) that fouls plugs. Confirmed by negative fuel trim and black, sooty plugs.

System too lean, bank 1 / bank 2. Suspect vacuum leaks, low fuel pressure, a dirty MAF, or an exhaust leak fooling the O₂ sensor.

System too rich, bank 1 / bank 2. Suspect high fuel pressure, leaking injectors, a restricted air filter, or a dirty MAF reading high.

Always reproduce and confirm the customer's drivability concern before testing, so you fix the actual problem and can verify the repair.

A transformer that steps battery voltage up to thousands of volts to jump the spark-plug gap.

The low-voltage (battery-side) part of the ignition system that the ECM or ignition control module switches on and off to trigger the coil.

The high-voltage side of the ignition system — coil secondary winding, plug wires (if any), and spark plugs.

The component (or ECM function) that switches the coil primary on and off to time the spark.

An ignition system with no distributor; coils are fired electronically using the crankshaft/camshaft position sensors.

One coil fires two paired plugs at once — one cylinder on compression (useful) and its partner on exhaust (wasted). No distributor.

An ignition design that mounts an individual coil directly on each spark plug, eliminating plug wires.

Delivers the high-voltage spark to the combustion chamber. Worn, fouled, or wrong-gap plugs cause misfire and poor performance.

The distance the spark must jump. Too wide overloads the coil and misfires; too narrow gives a weak spark. Set to the manufacturer's spec.

How fast a plug sheds heat. A hotter plug resists fouling at low loads; a colder plug resists pre-ignition at high loads.

A plug coated with carbon (black, dry — rich/misfire) or oil (wet, shiny — oil burning), causing a weak or no spark.

When the spark fires relative to piston position. Fired before TDC so peak combustion pressure arrives just after TDC.

Firing the spark earlier (more degrees before TDC) for power and economy. Too much advance causes detonation.

Firing the spark later (closer to or after TDC). The ECM retards timing via the knock sensor to stop detonation; too much wastes power and overheats the exhaust.

A sensor that detects detonation so the ECM can retard ignition timing to protect the engine.

Reports crankshaft speed and position; the ECM times the spark and injection from it. No signal usually means a no-start.

Tells the ECM which cylinder is on compression so coil-on-plug and sequential injection fire in the right order.

Move a coil from a misfiring cylinder to a good one. If the misfire follows the coil, the coil is bad; if it stays, suspect the plug, injector, or compression.

The coil's available voltage must exceed what the plug gap requires. A wide gap, lean mixture, or high compression raises required voltage and can cause misfire.

Plug wires use carbon cores with a resistance spec; too-high resistance weakens the spark and causes misfire. Cracked insulation lets spark leak (crossfire).

The time the coil primary is switched on to build its magnetic field (saturation). The ECM controls dwell electronically on modern systems.

High voltage jumping to ground or an adjacent wire instead of the plug, from cracked insulation or carbon tracking — causes intermittent misfire.

A misfire that appears mainly under acceleration. Suspect a weak coil, worn plugs (wide gap), or a secondary insulation breakdown that needs high voltage.

A burned conductive path on a coil-on-plug boot that lets spark leak to ground, causing a misfire. Replace the boot/coil and the plug.

The baseline spark timing the ECM starts from before adding electronic advance based on load, RPM, and knock.

Detonation heard mainly under load. Check for low octane, excess advance, a non-working EGR (high NOx/temp), carbon buildup, or a knock-sensor fault.

The valve that controls how much air enters the engine. On drive-by-wire systems the ECM controls it through a motor.

Cleans incoming air. A restricted filter richens the mixture and reduces power; replace on schedule.

Directly measures the weight of air entering the engine so the ECM can match fuel to it. A contaminated MAF reading low causes lean fuel trims.

Measures intake-manifold pressure (engine load); used with RPM and air temp to estimate airflow in speed-density systems.

Reports incoming air temperature so the ECM can adjust fuel and timing for air density.

Pressurizes and delivers fuel from the tank to the injectors. A weak pump causes low pressure, a lean condition, and hesitation under load.

Maintains the correct fuel pressure at the injectors. A leaking or stuck regulator causes a rich or lean condition and hard starting.

An electrically controlled valve that sprays a metered amount of fuel; pulse width (open time) sets the quantity.

How long the ECM holds an injector open. Longer pulse width = more fuel; the ECM lengthens it for enrichment and shortens it to lean the mixture.

An injector at each intake port. Often fired sequentially (timed to each cylinder's intake) for precise fuel control.

Fuel sprayed directly into the combustion chamber at high pressure, allowing precise control and higher efficiency.

Adds fuel to one cylinder — a rich single cylinder, hard hot start, and possible fouling or misfire.

Starves one cylinder of fuel — a lean single cylinder, rough idle, and a cylinder-specific misfire.

Reads leftover oxygen in the exhaust and tells the ECM whether the mixture burned rich or lean; swings ~0.1 V (lean) to ~0.9 V (rich) around 0.45 V.

Reports the actual air-fuel ratio over a wide range, not just rich/lean, allowing more precise fuel control than a conventional O₂ sensor.

The upstream (pre-cat) sensor controls fueling; the downstream (post-cat) sensor monitors catalytic-converter efficiency.

An exhaust device that converts CO and HC to CO₂ and water, and NOx to nitrogen and oxygen. Needs the engine near 14.7:1 to work.

Reduces exhaust noise and vibration. A collapsed muffler or internal baffle can restrict the exhaust and rob power.

A clogged converter or collapsed pipe that chokes the engine — power loss at higher RPM and a slowly dropping vacuum reading.

Lets outside air reach the O₂ sensor, which reads it as lean, causing the ECM to add fuel and run rich.

Connecting a gauge to verify the fuel system delivers the spec pressure under key-on, idle, and load — a first step for lean/rich and starting complaints.

How completely each cylinder fills with air. A restricted intake or exhaust lowers it and limits power.

Meters air around the throttle plate to set idle speed on cable-throttle engines. A stuck IAC causes a high, low, or unstable idle.

Reports throttle angle (driver demand) so the ECM can add fuel for acceleration. A glitchy TPS causes surge, stumble, or shift issues.

Extra fuel the ECM adds when cold (based on coolant temp) so a cold engine starts and runs; a faulty ECT sensor breaks it.

Mass Air Flow sensor — directly measures the weight of intake air for fuel calculation.

Manifold Absolute Pressure sensor — measures manifold pressure (engine load) to estimate airflow.

Intake Air Temperature sensor — reports incoming air temperature so the ECM adjusts for air density.

Idle Air Control valve — meters air around the throttle plate to set idle speed on cable-throttle engines.

Extra fuel the ECM adds (sensed by the TPS/MAP) during acceleration to prevent a lean stumble.

The ECM shuts off the injectors during closed-throttle deceleration to save fuel and cut emissions.

A heating element that brings the O₂ sensor to operating temperature quickly so closed loop starts sooner; its own OBD-II monitor checks it.

How often the O₂ sensor switches rich/lean per second; a lazy (slow) sensor that doesn't switch enough should be replaced.

Removing carbon from the throttle bore and plate to fix a high or unstable idle on a dirty drive-by-wire throttle body.

Power loss at higher RPM, possible overheating, a drop in vacuum as RPM rises, and high backpressure on a gauge test.

The three regulated tailpipe pollutants: hydrocarbons (unburned fuel), carbon monoxide (incomplete combustion), and oxides of nitrogen (high combustion temperature).

Unburned fuel in the exhaust. Raised by a rich mixture, misfire, or poor combustion. The catalytic converter oxidizes HC to CO₂ and water.

A product of incomplete combustion. Raised by a rich mixture. The catalytic converter oxidizes CO to CO₂.

Formed by high combustion temperature. Controlled by EGR (lowering combustion temperature) and the converter's reduction function.

Cleans all three regulated pollutants — oxidizes HC and CO, reduces NOx — but only when the engine is held near a 14.7:1 ratio.

Running rich for long periods overheats and melts the converter; oil, coolant, or leaded fuel can poison the catalyst.

Exhaust Gas Recirculation — routes inert exhaust into the intake to lower combustion temperature and cut NOx.

Floods the intake with exhaust at idle, causing rough idle, stalling, or hesitation.

Lets combustion temperature rise, raising NOx and allowing detonation (spark knock under load).

Positive Crankcase Ventilation — routes crankcase blow-by back to the intake to be burned, cutting HC and relieving crankcase pressure.

Adds unmetered air, leaning the idle and causing a vacuum-leak-like rough idle.

Builds crankcase pressure that pushes oil out through seals and gaskets and increases blow-by.

Evaporative emission control — captures fuel-tank vapors in a charcoal canister and purges them into the intake to be burned.

Stores fuel-tank vapors on activated charcoal until the ECM opens the purge valve to draw them into the engine.

The ECM-controlled valve that meters stored vapors from the canister into the intake during purge.

A loose or missing gas cap is the classic cause of a small EVAP leak code (P0455/P0457). The system self-tests for leaks.

Pumps fresh air into the exhaust on cold start to help the converter light off faster and burn excess HC and CO.

The temperature at which the converter starts working efficiently. Secondary air and quick warm-up strategies speed it up.

Monitors converter efficiency rather than controlling fuel; a downstream signal mirroring the upstream one means the converter is failing.

Indicate EGR flow or position faults, often from carbon-clogged passages or a stuck valve.

An EVAP feature that captures gasoline vapors during refueling at the vehicle instead of at the pump.

Combustion gases that slip past the rings into the crankcase. The PCV system burns them; excessive blow-by means worn rings or cylinders.

Measuring HC, CO, CO₂, O₂, and NOx at the tailpipe to judge combustion and emissions performance.

Indicates a rich mixture — too much fuel or too little air. Check fuel pressure, injectors, and air-intake restrictions.

Indicates unburned fuel — a misfire, ignition fault, lean misfire, or a failed converter.

Indicates high combustion temperature — a non-functioning EGR system, lean condition, overheating, or excess carbon (compression) and advance.

A state test of tailpipe emissions and/or OBD-II readiness and codes. Not-ready monitors or a commanded MIL can fail it.

Older emissions devices actuated by engine vacuum; broken or misrouted vacuum hoses are a common fault source.

Deposits, common on GDI engines, that restrict airflow and disturb fuel delivery, causing misfire, rough idle, and emissions faults.

Closed loop (warm, O₂ feedback) keeps emissions lowest; staying in open loop from a sensor fault raises emissions and fuel use.

Confirm an emissions device's fault with live data and a functional test before replacing it — many emissions codes are caused by wiring or vacuum issues.

The converter can only clean all three pollutants when the mixture oscillates tightly around λ = 1 (14.7:1) — that is the whole point of closed-loop control.

A faulty ECT reading cold keeps the engine rich and in open loop, raising HC and CO and preventing the catalyst monitor from running.

Measuring pressure ahead of the converter to detect a restriction (plugged converter or collapsed pipe) that limits power and raises emissions.

Exhaust Gas Recirculation — the emissions device that recirculates exhaust to lower combustion temperature and reduce NOx.

Evaporative emission control system — captures and burns fuel-tank vapors so they never reach the atmosphere.

Positive Crankcase Ventilation — routes crankcase blow-by back to the intake to be burned.

The Engine (or Powertrain) Control Module — the computer that reads the sensors and controls fuel, spark, idle, and emissions devices.

Electronic Control Unit — a generic term for an on-board computer module such as the ECM/PCM.

Inputs are the sensors the ECM reads (MAF, O₂, TPS, CKP); outputs are what it controls (injectors, coils, idle, EVAP purge). Diagnose by checking both.

The regulated 5-volt supply the ECM sends to many sensors; the sensor varies it, and the return signal tells the ECM the value.

A potentiometer reporting throttle angle. A dead spot or erratic signal causes surging, stumble, or a stored TPS code.

Reports engine speed and position; the ECM uses it for spark timing, injection, and misfire detection. No signal = crank-no-start.

Reports camshaft position so the ECM knows which cylinder is on compression for sequential injection and coil-on-plug timing.

Reports engine temperature. A sensor stuck cold keeps the engine in open loop and rich; one stuck hot can cause hard starting.

A primary fueling input. A dirty MAF reading low makes the ECM under-fuel, producing positive (lean) fuel trims.

Reports detonation to the ECM, which then retards timing. A failed knock sensor can set a code and force conservative timing.

On-Board Diagnostics II — the standardized federal system that monitors emissions systems, stores trouble codes, and turns on the MIL when a fault occurs.

A standardized fault code (e.g. P0301) identifying the system, whether generic or manufacturer-specific, the subsystem, and the specific fault.

P = powertrain, 0 = generic SAE, 3 = ignition/misfire subsystem, 01 = cylinder 1. So P0301 = a generic powertrain misfire on cylinder 1.

A code set on the first failed trip; it becomes a confirmed (stored) code only if the fault repeats on a second trip.

A code that has failed on two consecutive trips; the ECM commands the MIL on and stores the DTC.

A snapshot of operating conditions (RPM, load, coolant temp, fuel trim, speed) the ECM records the instant it sets a code — the first clue to chase.

The check-engine light. Steady = a stored emissions fault; flashing = an active misfire severe enough to damage the converter.

A self-test the OBD-II system runs on a system or component. Continuous monitors run constantly; non-continuous ones run when their drive cycle conditions are met.

Run constantly: the misfire monitor, fuel-system monitor, and comprehensive components monitor.

Run only when enabling conditions are met: catalyst, evaporative (EVAP), oxygen sensor, O₂ heater, EGR, and secondary-air monitors.

A specific pattern of driving conditions required to run the non-continuous monitors and set their readiness status to complete.

Whether each OBD-II monitor has run and completed since codes were cleared. Not-ready monitors can fail a state emissions inspection.

An OBD-II monitor that detects a cylinder failing to fire by watching small variations in crankshaft speed.

Watches fuel trim; sets a lean code (P0171/P0174) or rich code (P0172/P0175) when trim exceeds limits.

Compares downstream to upstream O₂ activity to judge converter efficiency; a lazy downstream signal means a good converter (P0420/P0430 if not).

Real-time sensor and output values the scan tool reads from the ECM (parameter IDs). Used to confirm a fault before replacing a part.

A device that reads codes, freeze-frame, live data, and readiness, and performs bidirectional tests — the core A8 diagnostic instrument.

A scan-tool function that commands an output (e.g. cycle the EVAP purge, fire an injector, command idle) to test a component actively.

The standardized 16-pin OBD-II connector (within reach of the driver) where the scan tool plugs in.

Generic (global) OBD-II data is standardized across makes; enhanced (manufacturer-specific) data gives more parameters and codes.

Erasing DTCs also resets monitors to not-ready. Clear only after confirming the repair, then drive a cycle so monitors re-run and confirm the fix.

Reproduce the original conditions (use freeze-frame), confirm the fault is gone and the monitor runs, and ensure no new codes return.

Comparing a suspect sensor's reading to a specification or a known-good vehicle to decide if it is actually faulty.

An open, short, or high-resistance connection that mimics a sensor failure. Check the circuit before condemning the sensor.

Catalyst efficiency below threshold (bank 1 / bank 2). Diagnosed by comparing upstream and downstream O₂ activity after ruling out exhaust leaks and rich conditions.

A fuel system that estimates airflow from MAP, RPM, and air temperature instead of a MAF sensor.

Used to pull the correct specifications, calibration, and service information for the exact vehicle being diagnosed.

Long-term corrections (like LTFT and idle adaptations) the ECM learns and stores; they may need relearning after certain repairs.

Reprogramming the ECM with updated calibration to fix a drivability or emissions concern addressed by a technical service bulletin.

A reduced-power strategy the ECM enters on certain faults, using default values to protect the engine and keep the vehicle drivable.

Throttle Position Sensor — reports throttle-plate angle (driver demand) to the ECM.

Crankshaft Position sensor — reports engine speed/position for spark timing, injection, and misfire detection.

Camshaft Position sensor — tells the ECM which cylinder is on compression for sequential firing.

Powertrain Control Module — the computer controlling the engine (and often transmission); the ECM is the engine portion.

Diagnostic Trouble Code — a standardized fault code stored by the ECM that identifies a circuit or symptom.

Malfunction Indicator Lamp — the check-engine light. Steady = stored fault; flashing = converter-damaging misfire.

Engine Coolant Temperature sensor — reports engine temperature, controlling cold enrichment and open/closed loop entry.

Electronic throttle control: a pedal-position sensor tells the ECM the request, and the ECM positions a motorized throttle body.

An ECM-controlled system that changes valve timing for power, economy, and emissions; faults can cause rough idle, codes, and a rattle.

Bank 1 is the cylinder bank containing cylinder 1; bank 2 is the opposite bank on a V engine. Codes use the bank to localize the fault.

Sensor 1 is upstream of the converter (controls fuel); sensor 2 is downstream (monitors the converter). Codes specify which.

Commanding a component on/off with a scan tool to confirm it responds — e.g. cycling the EVAP purge or EGR to verify operation.

Use freeze-frame, a recording (snapshot/movie) of live data, and wiggle/heat tests to catch a fault that doesn't set a hard code.

OBD-II Mode 06 test results behind each monitor (the actual measured values vs. limits) used to spot a system that is close to failing.