OBD-II Code Directory
OBD-II Code Lookup
Plain-English explanations for the most common diagnostic trouble codes. Trust-first guidance: inspect before you replace, and don't pay for parts you don't need.
How to use this directory
1. Find your code — search by code (e.g. P0420) or browse by family below.
2. Read the inspect-first guidance — every page tells you what to check before replacing parts.
3. Run a full scan — a single code rarely tells the whole story. The free scan reads every active code plus your vehicle's maintenance context.
Misfire Codes
Cylinder misfires (P0300–P0308). High urgency when the check-engine light flashes — unburned fuel can damage the catalytic converter.
Inspect ignition components before replacing parts. A coil-swap test isolates a bad coil in minutes.
→ Misfire diagnostic pathYour engine's control module detected random misfires across multiple cylinders — meaning unburned fuel is entering the exhaust.
The engine control module detected repeated misfires specifically in cylinder 1.
The engine control module detected repeated misfires specifically in cylinder 2.
The engine control module detected repeated misfires specifically in cylinder 3.
The engine control module detected repeated misfires specifically in cylinder 4.
The engine control module detected repeated misfires specifically in cylinder 5.
The engine control module detected repeated misfires specifically in cylinder 6.
The engine control module detected repeated misfires specifically in cylinder 7.
The engine control module detected repeated misfires specifically in cylinder 8.
The ECM detected one or more misfires within the first 1,000 revolutions after startup, before specific cylinder data was conclusive — usually accompanied or followed by a P030x cylinder-specific code.
Fuel & Air Codes
Lean / rich fuel-trim codes (P0171, P0172, P0174, P0175). Usually a vacuum leak or MAF sensor — rarely the fuel pump.
Check vacuum and MAF first. Fuel pumps are the last thing to replace, not the first.
The mass airflow sensor is reporting readings outside the expected range — often dirty or failing.
The mass airflow sensor signal voltage is below the ECM's expected minimum — usually a dirty sensor, an air leak, or a wiring fault.
The MAF sensor signal is above the ECM's expected maximum — usually a wiring fault, a contaminated sensor reading high, or a torn intake boot allowing air turbulence at the sensor.
The intake air temperature sensor is reading abnormally high — the ECM thinks incoming air is hotter than it actually is.
The throttle or pedal position sensor A signal does not match expected performance.
The throttle or pedal position sensor A signal is lower than expected.
The engine's fuel control system detected too much air and too little fuel in the mixture on Bank 1.
The engine is running too rich on Bank 1 — too much fuel and not enough air in the combustion mixture.
The ECM detected a fuel-trim control problem on Bank 2.
The engine is running too lean on Bank 2 — the Bank 2 counterpart to P0171.
The engine is running too rich on Bank 2 — the Bank 2 counterpart to P0172.
The ECM detected a fuel composition sensor circuit problem.
The fuel composition sensor signal is outside the expected range.
The fuel composition sensor signal is lower than expected.
The fuel composition sensor signal is higher than expected.
The post-catalyst fuel trim on Bank 1 is maxed out trying to compensate for lean operation — downstream fuel control is failing.
The upstream O2 sensor on Bank 1 is stuck reporting a permanently lean signal.
The Bank 1 upstream O2 sensor is reporting a rich condition the ECM cannot correct — usually an actual rich engine condition, sensor contamination, or a wiring fault.
The upstream oxygen sensor on Bank 1 is stuck reading rich — causing lean engine operation.
The Bank 2 upstream O2 sensor is reporting a rich condition the ECM cannot correct — the Bank 2 counterpart to P2196.
O2 Sensor & Emissions Codes
Oxygen-sensor and catalyst-efficiency codes (P0420, P0430, P0130–P0155, P0135). Many P0420 codes are misdiagnosed as bad converters.
Rule out exhaust leaks and upstream sensor faults before condemning the catalytic converter.
→ O2 sensor diagnostic pathThe upstream oxygen sensor on Bank 1 is sending an incorrect or erratic signal — the ECM cannot accurately control the air-fuel mixture.
The Bank 1 Sensor 1 oxygen sensor signal is lower than expected.
The Bank 1 Sensor 1 oxygen sensor signal is higher than expected.
The upstream oxygen sensor on Bank 1 is responding too slowly, impairing fuel control and efficiency.
The ECM detected little or no activity from the Bank 1 Sensor 1 oxygen sensor.
The heater circuit in the upstream oxygen sensor on Bank 1 has failed — the sensor takes much longer to reach operating temperature.
The ECM detected an oxygen sensor circuit problem for Bank 1 Sensor 2.
The Bank 1 Sensor 2 oxygen sensor signal is lower than expected.
The Bank 1 Sensor 2 oxygen sensor signal is higher than expected.
The Bank 1 Sensor 2 oxygen sensor response is slower than expected.
The ECM detected little or no activity from the Bank 1 Sensor 2 oxygen sensor.
The heater circuit in the downstream O2 sensor on Bank 1 is not drawing the expected current — usually a failed heater inside the sensor or a wiring/fuse issue.
The ECM detected an oxygen sensor circuit problem for Bank 1 Sensor 3.
The Bank 1 Sensor 3 oxygen sensor signal is lower than expected.
The Bank 1 Sensor 3 oxygen sensor signal is higher than expected.
The Bank 1 Sensor 3 oxygen sensor response is slower than expected.
The ECM detected little or no activity from the Bank 1 Sensor 3 oxygen sensor.
The Bank 1 Sensor 3 oxygen sensor heater circuit has a fault.
The ECM detected an oxygen sensor circuit problem for Bank 2 Sensor 1.
The Bank 2 Sensor 1 oxygen sensor signal is lower than expected.
The Bank 2 Sensor 1 oxygen sensor signal is higher than expected.
The Bank 2 Sensor 1 oxygen sensor response is slower than expected.
The ECM detected little or no activity from the Bank 2 Sensor 1 oxygen sensor.
The ECM detected a heater circuit fault for the Bank 2 Sensor 1 oxygen sensor.
The ECM detected an oxygen sensor circuit problem for Bank 2 Sensor 2.
The Bank 2 Sensor 2 oxygen sensor signal is lower than expected.
The Bank 2 Sensor 2 oxygen sensor signal is higher than expected.
The Bank 2 Sensor 2 oxygen sensor response is slower than expected.
The ECM detected little or no activity from the Bank 2 Sensor 2 oxygen sensor.
The Bank 2 Sensor 2 oxygen sensor heater circuit has a fault.
The ECM detected an exhaust gas recirculation flow problem.
The ECM detected insufficient EGR flow.
The ECM detected excessive EGR flow.
The oxygen sensors detected that the catalytic converter on Bank 1 is no longer cleaning exhaust gases efficiently.
The warm-up catalytic converter on Bank 1 is not reaching efficiency standards — closely related to P0420.
The main catalytic converter on Bank 1 is reading below its efficiency threshold — but, like P0420, this often points to upstream causes (O2 sensors, exhaust leaks, misfires) before the converter itself.
The catalytic converter on Bank 2 is no longer cleaning exhaust gases efficiently — the Bank 2 counterpart to P0420.
The warm-up catalytic converter on Bank 2 is not meeting efficiency standards.
The main catalytic converter on Bank 2 is reading below its efficiency threshold — the Bank 2 counterpart to P0422.
EVAP Codes
Fuel-vapor leak codes (P0440, P0442, P0446, P0455, P0456). Most fixes start with the gas cap.
Tighten or replace the gas cap first. Smoke testing is the gold standard for small leaks.
→ EVAP leak diagnostic pathA general malfunction was detected in the evaporative emission system — often overlaps with P0442 or P0455.
The ECM detected purge flow that does not match the commanded EVAP purge operation.
A small leak was detected in the EVAP system — harder to locate than large leaks but causes the same emissions test failure.
The ECM detected an electrical fault in the EVAP purge control valve circuit.
The ECM detected an open circuit in the EVAP purge control valve circuit.
The ECM detected a shorted EVAP purge control valve circuit.
The ECM detected a problem with the EVAP vent control circuit or vent valve operation.
The ECM detected an open circuit in the EVAP vent control circuit.
The ECM detected a shorted EVAP vent control circuit.
The ECM detected a problem in the EVAP vent valve or solenoid circuit.
The ECM detected an EVAP pressure sensor signal problem.
The EVAP pressure sensor signal is outside the expected range or response.
The EVAP pressure sensor signal is lower than expected.
The EVAP pressure sensor signal is higher than expected.
A large leak was detected in the evaporative emission system — the most common fix is a loose gas cap.
A very small leak was detected in the EVAP system — even harder to locate than P0442 but just as harmless.
The ECM detected an EVAP leak commonly associated with a loose, missing, or faulty fuel cap.
The ECM detected a fault in the fuel level sensor circuit.
Cooling System Codes
Cooling-system codes covering coolant temperature and thermostat (P0128, P0115–P0118), engine overtemperature (P0217), cooling-fan circuits (P0480/P0481), and thermostat heater control (P0597/P0598/P0599). Affects fuel economy, warm-up performance, and overheat risk.
Verify actual coolant temperature and fan operation with a scanner before replacing the thermostat, temp sensor, or fan relay. Stop driving immediately if overheating is suspected.
→ Cooling system diagnostic pathThe ECM detected an out-of-range or invalid signal from the engine coolant temperature (ECT) sensor — usually a wiring fault, connector issue, or a failed sensor.
The coolant temperature sensor signal is outside the expected range or does not match engine warm-up behavior.
The coolant temperature sensor signal voltage is too low — usually a wiring short to ground, a damaged sensor, or a connector issue.
The coolant temperature sensor signal voltage is too high — usually an open in the signal wire, a disconnected sensor, or a failed sensor reading open internally.
The engine is taking too long to reach its normal operating temperature — almost always a stuck-open thermostat.
The ECM has confirmed an actual engine overheat condition — pull over safely, shut the engine off, and let it cool before driving any further.
The ECM detected a problem in cooling fan 1 control circuit.
The ECM detected a problem in cooling fan 2 control circuit.
The ECM detected an open circuit or control fault in the thermostat heater control circuit.
The thermostat heater control circuit voltage is lower than expected.
The thermostat heater control circuit voltage is higher than expected.
Ignition / Timing / Sensor Codes
Variable-valve timing, cam/crank position, and timing-related codes (P0011, P0012, P0014, P0016, P0335, P0340).
Many of these are oil-related — check oil level, condition, and viscosity before replacing solenoids.
The intake camshaft on Bank 1 is advancing further than it should — oil quality is the leading cause.
The intake camshaft on Bank 1 is retarded beyond the ECM's commanded position — oil quality is the leading cause.
The exhaust camshaft on Bank 1 is over-advanced — the exhaust-side counterpart to P0011.
The crankshaft and intake camshaft positions on Bank 1 are not correlating correctly — often a timing chain issue.
The ECM detected camshaft timing over-advanced on Bank 2.
The Bank 2 intake camshaft timing is more retarded than commanded.
The engine control module detected a malfunction in the Bank 1 knock sensor circuit — without a working knock sensor, the ECM cannot protect the engine from detonation.
The knock sensor signal is lower than expected on Bank 1.
The knock sensor signal is higher than expected on Bank 1.
The engine control module lost the crankshaft position signal — the engine may stall or refuse to start.
The engine control module lost the camshaft position signal — needed for precise fuel injection timing.
The engine computer detected idle speed higher than expected in the idle control system.
Transmission Codes
Transmission control module codes (P0700 family). Often a symptom of another code stored in the TCM.
Always pull TCM-specific codes alongside P0700 — the U-prefix or P07xx codes tell the real story.
The transmission control module reported a fault — P0700 is an informational code that always accompanies a more specific transmission code.
The TCM/PCM received an invalid or inconsistent signal from the transmission range sensor, which tells the vehicle what gear is selected.
The transmission control system detected a problem with the input or turbine speed sensor circuit.
The transmission control system detected a problem with the output speed sensor circuit.
The TCM detected that the actual gear ratio does not match the commanded gear ratio, often from slipping, fluid, solenoid, or internal clutch issues.
The transmission control system detected a torque converter clutch control circuit problem.
The TCM detected a fault with shift solenoid A or its control circuit.
The TCM detected a fault with shift solenoid B or its control circuit.
Battery & Charging Codes
System voltage and charging codes (P0562 System Voltage Low, P0563 System Voltage High). Usually an alternator, battery, or wiring issue — not transmission-related.
Test battery state-of-charge and alternator output before replacing parts. Inspect terminals and grounds for corrosion.
Other / Generic
Codes that do not fall into the families above.
Use a scanner to confirm related codes before replacing parts.
The ECM detected that engine cam/crank position correlation on Bank 1 is outside the expected range.
The ECM detected that engine cam/crank position correlation on Bank 2 is outside the expected range.
The ECM detected a camshaft position actuator circuit fault on Bank 2.
The Bank 1 Sensor 1 oxygen sensor heater control circuit has a fault.
The Bank 1 Sensor 1 oxygen sensor heater control circuit voltage is lower than expected.
The Bank 1 Sensor 1 oxygen sensor heater control circuit voltage is higher than expected.
The Bank 1 Sensor 2 oxygen sensor heater control circuit has a fault.
The Bank 1 Sensor 2 oxygen sensor heater control circuit voltage is lower than expected.
The Bank 1 Sensor 2 oxygen sensor heater control circuit voltage is higher than expected.
The Bank 2 Sensor 1 oxygen sensor heater control circuit has a fault.
The Bank 2 Sensor 1 oxygen sensor heater control circuit voltage is lower than expected.
The Bank 2 Sensor 1 oxygen sensor heater control circuit voltage is higher than expected.
The Bank 2 Sensor 2 oxygen sensor heater control circuit has a fault.
The Bank 2 Sensor 2 oxygen sensor heater control circuit voltage is lower than expected.
The Bank 2 Sensor 2 oxygen sensor heater control circuit voltage is higher than expected.
The ECM detected that manifold pressure, airflow, and throttle position signals do not agree.
The ambient air temperature sensor signal is higher than expected.
The MAP or barometric pressure sensor signal is outside the expected range or does not match engine conditions.
The MAP or barometric pressure sensor signal is lower than expected.
The MAP or barometric pressure sensor signal is higher than expected.
The intake air temperature sensor signal is outside the expected range or does not respond normally.
The intake air temperature sensor signal is lower than expected.
The throttle or pedal position sensor A circuit is outside the expected range.
The throttle or pedal position sensor A signal is higher than expected.
The ECM detected a fuel-trim control problem on Bank 1.
The ECM detected a general fault in one or more fuel injector circuits.
The ECM detected an injector circuit fault on cylinder 1.
The ECM detected an injector circuit fault on cylinder 2.
The ECM detected an injector circuit fault on cylinder 3.
The ECM detected an injector circuit fault on cylinder 4.
The ECM detected an injector circuit fault on cylinder 5.
The ECM detected an injector circuit fault on cylinder 6.
The engine computer detected an open or electrical fault in the fuel injector circuit for cylinder 7.
The engine computer detected an open or electrical fault in the fuel injector circuit for cylinder 8.
The throttle or pedal position sensor B circuit is outside the expected range.
The control module detected a problem in the fuel pump primary control circuit, often involving the relay, fuse, wiring, or pump command circuit.
The fuel pump secondary circuit voltage is lower than expected, commonly from wiring resistance, a weak relay, connector corrosion, or pump circuit fault.
The fuel pump secondary circuit voltage is higher than expected, usually from a stuck relay, short to voltage, or control circuit issue.
The engine computer detected boost pressure higher than the safe commanded limit.
The engine computer detected less boost pressure than expected from the turbocharger or supercharger system.
The ECM detected a fault in ignition coil A primary or secondary circuit.
The ECM detected a fault in ignition coil B primary or secondary circuit.
The ECM detected a fault in ignition coil C primary or secondary circuit.
The ECM detected a fault in ignition coil D primary or secondary circuit.
The ECM detected a fault in ignition coil E primary or secondary circuit.
The ECM detected a fault in ignition coil F primary or secondary circuit.
The ECM detected a fault in ignition coil G primary or secondary circuit.
The ECM detected a fault in ignition coil H primary or secondary circuit.
The ECM detected a vehicle speed sensor signal problem.
The ECM detected an idle control system fault.
The engine computer detected idle speed lower than expected in the idle control system.
The control module detected a serial communication link fault between vehicle modules.
The PCM/ECM detected an internal memory checksum problem or corrupted memory state.
The control module detected a programming or calibration error.
The PCM/ECM is not retaining keep-alive memory correctly, often from voltage, ground, or module memory issues.
The PCM/ECM detected an internal random-access memory fault or related module integrity issue.
The PCM/ECM detected an internal read-only memory fault or related module integrity issue.
Don't guess — scan your vehicle
A single OBD2 code rarely tells the whole story. The free Maintenance Scan reads every active code, factors in your mileage and climate, and prioritizes what to fix first.