- Engine performance (drivability)
- 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.
- Open loop
- 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.
- Closed loop
- 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.
- Stoichiometric ratio
- 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.
- Lambda (λ)
- Air-fuel ratio relative to stoichiometric: λ = 1 is 14.7:1, λ < 1 is rich, λ > 1 is lean.
- Fuel trim
- The percentage the ECM adjusts fuel from its base calculation. Positive = adding fuel (lean correction); negative = removing fuel (rich correction).
- Short-term fuel trim (STFT)
- The instant fuel correction the ECM makes in response to the oxygen sensor, swinging up and down moment to moment.
- Long-term fuel trim (LTFT)
- The learned, averaged fuel correction stored over time. A large LTFT signals a persistent rich or lean condition.
- Positive fuel trim
- 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.
- Negative fuel trim
- 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.
- Lean condition
- Too much air or too little fuel (λ > 1). Causes positive fuel trim, possible misfire, and lean codes (P0171/P0174).
- Rich condition
- Too much fuel or too little air (λ < 1). Causes negative fuel trim, black smoke, fouled plugs, and rich codes (P0172/P0175).
- Vacuum leak (unmetered air)
- Air entering after the MAF that the ECM didn't measure. Leans the mixture — worse at idle (high positive trim), better at higher RPM.
- Misfire
- A cylinder failing to fire properly — from ignition, fuel, or compression. Causes rough idle, lost power, and a flashing or steady MIL.
- Flashing check-engine light (MIL)
- An active misfire severe enough to damage the catalytic converter. Stop driving and diagnose the misfire immediately.
- Steady check-engine light (MIL)
- A stored emissions-related fault that is not an immediate threat. Retrieve the code and diagnose.
- Random misfire (P0300)
- A misfire across multiple cylinders, pointing to a shared cause like a vacuum leak, low fuel pressure, or contaminated fuel.
- Cylinder-specific misfire (P0301–P0312)
- A misfire isolated to one cylinder, pointing to that cylinder's spark plug, coil, injector, or a compression problem.
- Detonation (knock / ping)
- Uncontrolled, spontaneous combustion AFTER the spark. Caused by low octane, too much advance, lean/hot mixture, carbon, or high compression.
- Pre-ignition
- The mixture igniting BEFORE the spark, from a hot spot such as a glowing carbon deposit or an overheated spark plug.
- White exhaust smoke
- Coolant entering the combustion chamber — usually a blown head gasket or cracked head. (Thin cold-start vapor is normal.)
- Blue exhaust smoke
- Oil burning in the cylinder — worn rings, worn valve guides, or bad valve stem seals.
- Black exhaust smoke
- A rich air-fuel mixture — too much fuel or too little air (clogged air filter, leaking injector, faulty sensor).
- Hesitation / stumble
- A momentary loss of power on acceleration. Often a lean transition, a weak accelerator-pump function, low fuel pressure, or a TPS fault.
- Surging
- A repeated rise and fall in engine speed at steady throttle. Causes include a lean condition, EGR fault, contaminated MAF, or a faulty TPS.
- Dieseling (run-on)
- An engine that keeps running after the key is off, from hot spots or carbon deposits igniting the mixture without spark.
- Hard start (hot)
- Difficult starting when warm. Suspect a leaking injector flooding the engine, a hot-soak fuel-pressure loss, or a coolant-temp sensor fault.
- Hard start (cold)
- Difficult starting when cold. Suspect low fuel pressure, a faulty coolant-temp sensor (no cold enrichment), weak ignition, or low compression.
- Rough idle
- Unstable idle. Causes include vacuum leaks, a stuck-open EGR, a misfire, a dirty throttle body, or an IAC/idle-control fault.
- Vacuum gauge test
- Reads manifold vacuum at idle. Steady low = vacuum leak; fluctuating = burned valve; slowly dropping at higher RPM = restricted exhaust.
- Power-balance test
- Disables one cylinder at a time and watches RPM. Little or no RPM drop = that cylinder is weak or dead.
- Compression test
- 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.
- Cylinder leak-down test
- Applies compressed air to a cylinder at TDC and reads the percent leaking; locates the leak (valves, rings, or head gasket).
- Top dead center (TDC)
- The highest point of piston travel. Many ignition-timing and diagnostic references use a cylinder at TDC on the compression stroke.
- Engine vacuum
- The pressure difference below atmospheric in the intake manifold; a key drivability clue and the power source for many emissions devices.
- No-start (cranks, won't fire)
- The engine cranks but doesn't start. Check for spark, fuel, and a crank position signal — a dead CKP sensor is a classic cause.
- Symptom vs. cause
- ASE A8 rewards diagnosing the underlying cause, not just the symptom or the code. Confirm with live data before replacing a part.
- Drivability complaint
- Any concern with how the engine runs — stalling, hesitation, misfire, poor economy, or a check-engine light. The starting point of A8 diagnosis.
- Intake manifold leak
- A gasket or boot leak that lets in unmetered air. Causes a lean condition and a vacuum-leak-like rough idle.
- Smoke test (intake)
- 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.
- Ignition coil
- A transformer that steps battery voltage up to thousands of volts to jump the spark-plug gap.
- Primary circuit
- 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.
- Secondary circuit
- The high-voltage side of the ignition system — coil secondary winding, plug wires (if any), and spark plugs.
- Ignition control module (ICM)
- The component (or ECM function) that switches the coil primary on and off to time the spark.
- Distributorless ignition (DIS)
- An ignition system with no distributor; coils are fired electronically using the crankshaft/camshaft position sensors.
- Wasted-spark ignition
- One coil fires two paired plugs at once — one cylinder on compression (useful) and its partner on exhaust (wasted). No distributor.
- Coil-on-plug (COP)
- An ignition design that mounts an individual coil directly on each spark plug, eliminating plug wires.
- Spark plug
- Delivers the high-voltage spark to the combustion chamber. Worn, fouled, or wrong-gap plugs cause misfire and poor performance.
- Spark plug gap
- 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.
- Spark plug heat range
- How fast a plug sheds heat. A hotter plug resists fouling at low loads; a colder plug resists pre-ignition at high loads.
- Fouled spark plug
- A plug coated with carbon (black, dry — rich/misfire) or oil (wet, shiny — oil burning), causing a weak or no spark.
- Ignition timing
- When the spark fires relative to piston position. Fired before TDC so peak combustion pressure arrives just after TDC.
- Spark advance
- Firing the spark earlier (more degrees before TDC) for power and economy. Too much advance causes detonation.
- Spark retard
- 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.
- Knock sensor
- A sensor that detects detonation so the ECM can retard ignition timing to protect the engine.
- Crankshaft position sensor (CKP) — ignition role
- Reports crankshaft speed and position; the ECM times the spark and injection from it. No signal usually means a no-start.
- Camshaft position sensor (CMP) — ignition role
- Tells the ECM which cylinder is on compression so coil-on-plug and sequential injection fire in the right order.
- Coil swap test
- 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.
- Available voltage vs. required voltage
- 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.
- Spark plug wire (secondary) resistance
- Plug wires use carbon cores with a resistance spec; too-high resistance weakens the spark and causes misfire. Cracked insulation lets spark leak (crossfire).
- Dwell
- The time the coil primary is switched on to build its magnetic field (saturation). The ECM controls dwell electronically on modern systems.
- Crossfire / spark leakage
- High voltage jumping to ground or an adjacent wire instead of the plug, from cracked insulation or carbon tracking — causes intermittent misfire.
- Engine misfire under load
- A misfire that appears mainly under acceleration. Suspect a weak coil, worn plugs (wide gap), or a secondary insulation breakdown that needs high voltage.
- Carbon-tracked coil boot
- 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.
- Initial (base) timing
- The baseline spark timing the ECM starts from before adding electronic advance based on load, RPM, and knock.
- Throttle body
- The valve that controls how much air enters the engine. On drive-by-wire systems the ECM controls it through a motor.
- Air filter
- Cleans incoming air. A restricted filter richens the mixture and reduces power; replace on schedule.
- Mass air flow (MAF) sensor
- 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.
- Manifold absolute pressure (MAP) sensor
- Measures intake-manifold pressure (engine load); used with RPM and air temp to estimate airflow in speed-density systems.
- Intake air temperature (IAT) sensor
- Reports incoming air temperature so the ECM can adjust fuel and timing for air density.
- Fuel pump
- Pressurizes and delivers fuel from the tank to the injectors. A weak pump causes low pressure, a lean condition, and hesitation under load.
- Fuel pressure regulator
- Maintains the correct fuel pressure at the injectors. A leaking or stuck regulator causes a rich or lean condition and hard starting.
- Fuel injector
- An electrically controlled valve that sprays a metered amount of fuel; pulse width (open time) sets the quantity.
- Injector pulse width
- 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.
- Multiport fuel injection (MPFI)
- An injector at each intake port. Often fired sequentially (timed to each cylinder's intake) for precise fuel control.
- Gasoline direct injection (GDI)
- Fuel sprayed directly into the combustion chamber at high pressure, allowing precise control and higher efficiency.
- Leaking / stuck-open injector
- Adds fuel to one cylinder — a rich single cylinder, hard hot start, and possible fouling or misfire.
- Clogged / restricted injector
- Starves one cylinder of fuel — a lean single cylinder, rough idle, and a cylinder-specific misfire.
- Oxygen (O₂) sensor
- 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.
- Wide-band air-fuel (A-F) sensor
- Reports the actual air-fuel ratio over a wide range, not just rich/lean, allowing more precise fuel control than a conventional O₂ sensor.
- Upstream vs. downstream O₂ sensor
- The upstream (pre-cat) sensor controls fueling; the downstream (post-cat) sensor monitors catalytic-converter efficiency.
- Catalytic converter
- 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.
- Muffler
- Reduces exhaust noise and vibration. A collapsed muffler or internal baffle can restrict the exhaust and rob power.
- Restricted (plugged) exhaust
- A clogged converter or collapsed pipe that chokes the engine — power loss at higher RPM and a slowly dropping vacuum reading.
- Exhaust leak before the O₂ sensor
- Lets outside air reach the O₂ sensor, which reads it as lean, causing the ECM to add fuel and run rich.
- Fuel pressure test
- 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.
- Volumetric efficiency
- How completely each cylinder fills with air. A restricted intake or exhaust lowers it and limits power.
- Idle air control (IAC) valve
- Meters air around the throttle plate to set idle speed on cable-throttle engines. A stuck IAC causes a high, low, or unstable idle.
- Throttle position sensor (TPS) — fuel role
- Reports throttle angle (driver demand) so the ECM can add fuel for acceleration. A glitchy TPS causes surge, stumble, or shift issues.
- Cold-start enrichment
- Extra fuel the ECM adds when cold (based on coolant temp) so a cold engine starts and runs; a faulty ECT sensor breaks it.
- ECM / PCM
- The Engine (or Powertrain) Control Module — the computer that reads the sensors and controls fuel, spark, idle, and emissions devices.
- ECU
- Electronic Control Unit — a generic term for an on-board computer module such as the ECM/PCM.
- Input vs. output
- 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.
- Reference voltage (5 V)
- The regulated 5-volt supply the ECM sends to many sensors; the sensor varies it, and the return signal tells the ECM the value.
- Throttle position sensor (TPS) — input
- A potentiometer reporting throttle angle. A dead spot or erratic signal causes surging, stumble, or a stored TPS code.
- Crankshaft position sensor (CKP)
- Reports engine speed and position; the ECM uses it for spark timing, injection, and misfire detection. No signal = crank-no-start.
- Camshaft position sensor (CMP)
- Reports camshaft position so the ECM knows which cylinder is on compression for sequential injection and coil-on-plug timing.
- Engine coolant temperature (ECT) sensor
- Reports engine temperature. A sensor stuck cold keeps the engine in open loop and rich; one stuck hot can cause hard starting.
- Mass air flow (MAF) — input role
- A primary fueling input. A dirty MAF reading low makes the ECM under-fuel, producing positive (lean) fuel trims.
- Knock sensor — input role
- Reports detonation to the ECM, which then retards timing. A failed knock sensor can set a code and force conservative timing.
- OBD-II
- On-Board Diagnostics II — the standardized federal system that monitors emissions systems, stores trouble codes, and turns on the MIL when a fault occurs.
- Diagnostic trouble code (DTC)
- A standardized fault code (e.g. P0301) identifying the system, whether generic or manufacturer-specific, the subsystem, and the specific fault.
- Reading a DTC (P0301)
- P = powertrain, 0 = generic SAE, 3 = ignition/misfire subsystem, 01 = cylinder 1. So P0301 = a generic powertrain misfire on cylinder 1.
- Pending code
- A code set on the first failed trip; it becomes a confirmed (stored) code only if the fault repeats on a second trip.
- Confirmed (stored) code
- A code that has failed on two consecutive trips; the ECM commands the MIL on and stores the DTC.
- Freeze-frame data
- 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.
- Malfunction indicator lamp (MIL)
- The check-engine light. Steady = a stored emissions fault; flashing = an active misfire severe enough to damage the converter.
- Monitor
- 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.
- Continuous monitors
- Run constantly: the misfire monitor, fuel-system monitor, and comprehensive components monitor.
- Non-continuous monitors
- Run only when enabling conditions are met: catalyst, evaporative (EVAP), oxygen sensor, O₂ heater, EGR, and secondary-air monitors.
- Drive cycle
- A specific pattern of driving conditions required to run the non-continuous monitors and set their readiness status to complete.
- Readiness (monitor) status
- Whether each OBD-II monitor has run and completed since codes were cleared. Not-ready monitors can fail a state emissions inspection.
- Misfire monitor
- An OBD-II monitor that detects a cylinder failing to fire by watching small variations in crankshaft speed.
- Fuel-system monitor
- Watches fuel trim; sets a lean code (P0171/P0174) or rich code (P0172/P0175) when trim exceeds limits.
- Catalyst monitor
- Compares downstream to upstream O₂ activity to judge converter efficiency; a lazy downstream signal means a good converter (P0420/P0430 if not).
- Live data (data stream / PIDs)
- Real-time sensor and output values the scan tool reads from the ECM (parameter IDs). Used to confirm a fault before replacing a part.
- Scan tool
- A device that reads codes, freeze-frame, live data, and readiness, and performs bidirectional tests — the core A8 diagnostic instrument.
- Bidirectional control
- A scan-tool function that commands an output (e.g. cycle the EVAP purge, fire an injector, command idle) to test a component actively.
- Data Link Connector (DLC)
- The standardized 16-pin OBD-II connector (within reach of the driver) where the scan tool plugs in.
- Generic vs. enhanced data
- Generic (global) OBD-II data is standardized across makes; enhanced (manufacturer-specific) data gives more parameters and codes.
- Clearing codes (and why to wait)
- 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.
- Verifying a repair
- Reproduce the original conditions (use freeze-frame), confirm the fault is gone and the monitor runs, and ensure no new codes return.
- Known-good comparison
- Comparing a suspect sensor's reading to a specification or a known-good vehicle to decide if it is actually faulty.
- Wiring / circuit fault
- An open, short, or high-resistance connection that mimics a sensor failure. Check the circuit before condemning the sensor.
- P0420 / P0430
- Catalyst efficiency below threshold (bank 1 / bank 2). Diagnosed by comparing upstream and downstream O₂ activity after ruling out exhaust leaks and rich conditions.
- Speed-density system
- A fuel system that estimates airflow from MAP, RPM, and air temperature instead of a MAF sensor.
- Vehicle Identification Number (VIN) in diagnosis
- Used to pull the correct specifications, calibration, and service information for the exact vehicle being diagnosed.
- Adaptive (learned) values
- Long-term corrections (like LTFT and idle adaptations) the ECM learns and stores; they may need relearning after certain repairs.
- ECM software update / reflash
- Reprogramming the ECM with updated calibration to fix a drivability or emissions concern addressed by a technical service bulletin.
- Limp-in (failsafe) mode
- A reduced-power strategy the ECM enters on certain faults, using default values to protect the engine and keep the vehicle drivable.
- HC, CO, NOx
- The three regulated tailpipe pollutants: hydrocarbons (unburned fuel), carbon monoxide (incomplete combustion), and oxides of nitrogen (high combustion temperature).
- Hydrocarbons (HC)
- Unburned fuel in the exhaust. Raised by a rich mixture, misfire, or poor combustion. The catalytic converter oxidizes HC to CO₂ and water.
- Carbon monoxide (CO)
- A product of incomplete combustion. Raised by a rich mixture. The catalytic converter oxidizes CO to CO₂.
- Oxides of nitrogen (NOx)
- Formed by high combustion temperature. Controlled by EGR (lowering combustion temperature) and the converter's reduction function.
- Three-way catalytic converter
- Cleans all three regulated pollutants — oxidizes HC and CO, reduces NOx — but only when the engine is held near a 14.7:1 ratio.
- Converter overheating / poisoning
- Running rich for long periods overheats and melts the converter; oil, coolant, or leaded fuel can poison the catalyst.
- EGR system
- Exhaust Gas Recirculation — routes inert exhaust into the intake to lower combustion temperature and cut NOx.
- Stuck-open EGR valve
- Floods the intake with exhaust at idle, causing rough idle, stalling, or hesitation.
- Stuck-closed / clogged EGR valve
- Lets combustion temperature rise, raising NOx and allowing detonation (spark knock under load).
- PCV system
- Positive Crankcase Ventilation — routes crankcase blow-by back to the intake to be burned, cutting HC and relieving crankcase pressure.
- Stuck-open PCV valve
- Adds unmetered air, leaning the idle and causing a vacuum-leak-like rough idle.
- Stuck-closed PCV valve
- Builds crankcase pressure that pushes oil out through seals and gaskets and increases blow-by.
- EVAP system
- Evaporative emission control — captures fuel-tank vapors in a charcoal canister and purges them into the intake to be burned.
- Charcoal (vapor) canister
- Stores fuel-tank vapors on activated charcoal until the ECM opens the purge valve to draw them into the engine.
- EVAP purge valve
- The ECM-controlled valve that meters stored vapors from the canister into the intake during purge.
- EVAP leak (loose gas cap)
- A loose or missing gas cap is the classic cause of a small EVAP leak code (P0455/P0457). The system self-tests for leaks.
- Secondary air injection
- Pumps fresh air into the exhaust on cold start to help the converter light off faster and burn excess HC and CO.
- Catalyst light-off
- The temperature at which the converter starts working efficiently. Secondary air and quick warm-up strategies speed it up.
- Downstream O₂ sensor (emissions role)
- Monitors converter efficiency rather than controlling fuel; a downstream signal mirroring the upstream one means the converter is failing.
- P0400-series (EGR) codes
- Indicate EGR flow or position faults, often from carbon-clogged passages or a stuck valve.
- Onboard refueling vapor recovery (ORVR)
- An EVAP feature that captures gasoline vapors during refueling at the vehicle instead of at the pump.
- Crankcase blow-by
- Combustion gases that slip past the rings into the crankcase. The PCV system burns them; excessive blow-by means worn rings or cylinders.
- Tailpipe gas analysis
- Measuring HC, CO, CO₂, O₂, and NOx at the tailpipe to judge combustion and emissions performance.
- High CO at idle
- Indicates a rich mixture — too much fuel or too little air. Check fuel pressure, injectors, and air-intake restrictions.
- High HC reading
- Indicates unburned fuel — a misfire, ignition fault, lean misfire, or a failed converter.
- High NOx reading
- Indicates high combustion temperature — a non-functioning EGR system, lean condition, overheating, or excess carbon (compression) and advance.
- Emissions inspection / smog test
- A state test of tailpipe emissions and/or OBD-II readiness and codes. Not-ready monitors or a commanded MIL can fail it.
- Vacuum-operated emissions controls
- Older emissions devices actuated by engine vacuum; broken or misrouted vacuum hoses are a common fault source.
- Carbon buildup (intake/valves)
- Deposits, common on GDI engines, that restrict airflow and disturb fuel delivery, causing misfire, rough idle, and emissions faults.
- Closed vs. open emissions loop
- Closed loop (warm, O₂ feedback) keeps emissions lowest; staying in open loop from a sensor fault raises emissions and fuel use.
- Verify before condemning (emissions)
- 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.
- λ = 1 and the converter
- 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.
- Coolant temp sensor and emissions
- A faulty ECT reading cold keeps the engine rich and in open loop, raising HC and CO and preventing the catalyst monitor from running.
- Exhaust backpressure test
- Measuring pressure ahead of the converter to detect a restriction (plugged converter or collapsed pipe) that limits power and raises emissions.
- Technician A / Technician B
- The signature ASE question format presenting two statements; you decide whether A only, B only, both, or neither is correct.
- Reading 'EGR' in context
- Exhaust Gas Recirculation — the emissions device that recirculates exhaust to lower combustion temperature and reduce NOx.
- Reading 'EVAP' in context
- Evaporative emission control system — captures and burns fuel-tank vapors so they never reach the atmosphere.
- Reading 'MAF' in context
- Mass Air Flow sensor — directly measures the weight of intake air for fuel calculation.
- Reading 'MAP' in context
- Manifold Absolute Pressure sensor — measures manifold pressure (engine load) to estimate airflow.
- Reading 'TPS' in context
- Throttle Position Sensor — reports throttle-plate angle (driver demand) to the ECM.
- Reading 'CKP' in context
- Crankshaft Position sensor — reports engine speed/position for spark timing, injection, and misfire detection.
- Reading 'CMP' in context
- Camshaft Position sensor — tells the ECM which cylinder is on compression for sequential firing.
- Reading 'PCM' in context
- Powertrain Control Module — the computer controlling the engine (and often transmission); the ECM is the engine portion.
- Reading 'STFT / LTFT' in context
- Short-Term and Long-Term Fuel Trim — the instant and learned fuel corrections; positive = adding fuel (lean), negative = removing fuel (rich).
- Reading 'DTC' in context
- Diagnostic Trouble Code — a standardized fault code stored by the ECM that identifies a circuit or symptom.
- Reading 'MIL' in context
- Malfunction Indicator Lamp — the check-engine light. Steady = stored fault; flashing = converter-damaging misfire.
- Reading 'PCV' in context
- Positive Crankcase Ventilation — routes crankcase blow-by back to the intake to be burned.
- Reading 'IAT' in context
- Intake Air Temperature sensor — reports incoming air temperature so the ECM adjusts for air density.
- Reading 'ECT' in context
- Engine Coolant Temperature sensor — reports engine temperature, controlling cold enrichment and open/closed loop entry.
- Reading 'IAC' in context
- Idle Air Control valve — meters air around the throttle plate to set idle speed on cable-throttle engines.
- Acceleration enrichment
- Extra fuel the ECM adds (sensed by the TPS/MAP) during acceleration to prevent a lean stumble.
- Deceleration fuel cutoff (DFCO)
- The ECM shuts off the injectors during closed-throttle deceleration to save fuel and cut emissions.
- Power enrichment (open loop)
- At wide-open throttle the ECM richens the mixture for power and ignores the O₂ sensor (temporary open loop).
- Lean misfire
- A misfire from too little fuel (low fuel pressure, vacuum leak, lean injector). Raises HC; confirmed by positive fuel trim plus a misfire code.
- Rich misfire / fouling
- A misfire from too much fuel (leaking injector, high pressure) that fouls plugs. Confirmed by negative fuel trim and black, sooty plugs.
- O₂ sensor heater
- A heating element that brings the O₂ sensor to operating temperature quickly so closed loop starts sooner; its own OBD-II monitor checks it.
- O₂ sensor cross-counts
- How often the O₂ sensor switches rich/lean per second; a lazy (slow) sensor that doesn't switch enough should be replaced.
- Throttle body (carbon) cleaning
- Removing carbon from the throttle bore and plate to fix a high or unstable idle on a dirty drive-by-wire throttle body.
- Drive-by-wire (ETC)
- Electronic throttle control: a pedal-position sensor tells the ECM the request, and the ECM positions a motorized throttle body.
- Variable valve timing (VVT)
- An ECM-controlled system that changes valve timing for power, economy, and emissions; faults can cause rough idle, codes, and a rattle.
- Engine knock under acceleration
- 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.
- Plugged converter symptoms
- Power loss at higher RPM, possible overheating, a drop in vacuum as RPM rises, and high backpressure on a gauge test.
- Lean code (P0171 / P0174)
- System too lean, bank 1 / bank 2. Suspect vacuum leaks, low fuel pressure, a dirty MAF, or an exhaust leak fooling the O₂ sensor.
- Rich code (P0172 / P0175)
- System too rich, bank 1 / bank 2. Suspect high fuel pressure, leaking injectors, a restricted air filter, or a dirty MAF reading high.
- Bank 1 vs. bank 2
- 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 vs. sensor 2 (O₂)
- Sensor 1 is upstream of the converter (controls fuel); sensor 2 is downstream (monitors the converter). Codes specify which.
- Functional (actuator) test
- Commanding a component on/off with a scan tool to confirm it responds — e.g. cycling the EVAP purge or EGR to verify operation.
- Intermittent fault diagnosis
- 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.
- Mode 06 data (OBD-II)
- 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.
- Verify the complaint first
- Always reproduce and confirm the customer's drivability concern before testing, so you fix the actual problem and can verify the repair.
