OBD2 Trouble Codes by System
Every OBD2 code we cover, grouped by system: what it means, the likely causes ranked by probability, and how to diagnose it.
Fuel and air
- P0087 Code Meaning: Fuel Rail/System Pressure Too LowP0087 means actual fuel rail pressure is below commanded pressure. Common causes: clogged filter, weak pump, restricted lines, or leaking regulator.
- P0088 Code Meaning: Fuel Rail/System Pressure Too HighP0088 means actual fuel rail pressure exceeds commanded pressure. Learn causes, symptoms, and diagnostic steps for high fuel pressure codes.
- P0171 Code Meaning: System Too Lean (Bank 1)P0171 means your engine is running with too much air or too little fuel on Bank 1. Learn the common causes, symptoms, and diagnostic steps.
- P0172 Code Meaning: System Too Rich (Bank 1)P0172 means the engine is running too rich on Bank 1—more fuel than air. Learn symptoms, common causes, and the diagnostic path to fix it.
- P0174 Code Meaning: System Too Lean (Bank 2)P0174 means the engine is running too lean on bank 2. Learn common causes, symptoms, and the diagnostic path—especially when paired with P0171.
- P0175 Code Meaning: System Too Rich (Bank 2)P0175 means the engine control unit detected too much fuel or too little air in Bank 2. Learn symptoms, common causes, and the diagnostic path.
- P0299 Code Meaning: Turbocharger/Supercharger UnderboostP0299 means your turbo or supercharger is producing less boost than commanded. Learn the common causes, symptoms, and diagnostic steps.
- P0505 Code: Idle Air Control System Malfunction MeaningP0505 means the ECM cannot maintain target idle speed. Learn symptoms, common causes like carbon buildup and vacuum leaks, and the diagnostic path.
- P0506 Code: Idle RPM Lower Than Expected Meaning & DiagnosisP0506 means idle speed is below the commanded range. Learn symptoms, common causes like carbon buildup and IAC valve faults, and the diagnostic path.
- P0507 Code Meaning: Idle RPM Higher Than ExpectedP0507 means idle RPM is higher than expected. Learn the most common causes—vacuum leaks, carbon buildup, IAC valve faults—and the diagnostic path.
Ignition, timing and misfire
- P0011 Code Meaning: Camshaft Position Timing Over-AdvancedP0011 means Bank 1 variable valve timing isn't reaching its commanded position. Low or dirty oil is the leading cause; a stuck valve or phaser is next.
- P0016 Code Meaning: Cam/Crank Position CorrelationP0016 means the crankshaft and camshaft position sensors disagree. Common causes: jumped timing chain, worn phaser, or failed VVT solenoid.
- P0300 Code Meaning: Random/Multiple Cylinder MisfireP0300 means random or multiple cylinder misfire detected. A flashing check engine light signals severe misfire that can damage the catalytic converter.
- P0301 Code Meaning: Cylinder 1 Misfire DetectedP0301 means your engine's first cylinder is misfiring. Learn what causes it, how to diagnose it by swapping parts, and when to call a mechanic.
- P0302 Code Meaning: Cylinder 2 Misfire DetectedP0302 means Cylinder 2 Misfire Detected. Learn symptoms, ranked causes (coil, plug, injector, compression), and the diagnostic path to find the fault.
- P0303 Code Meaning: Cylinder 3 Misfire DetectedP0303 means the engine control module detected a misfire in cylinder 3. Learn symptoms, common causes ranked by likelihood, and the diagnostic path.
- P0304 Code Meaning: Cylinder 4 Misfire DetectedP0304 means the engine control module detected a misfire in cylinder 4. Learn common causes, symptoms, and the diagnostic path to fix it.
- P0305 Code Meaning: Cylinder 5 Misfire DetectedP0305 means Cylinder 5 Misfire Detected. Learn symptoms, common causes ranked by likelihood, and the diagnostic path to find the fault.
- P0306 Code Meaning: Cylinder 6 Misfire DetectedP0306 means the engine control module detected a misfire in cylinder 6. Learn the symptoms, common causes ranked by likelihood, and the diagnostic path.
- P0307 Code Meaning: Cylinder 7 Misfire DetectedP0307 means Cylinder 7 Misfire Detected. Learn what causes cylinder 7 misfires, symptoms to watch for, and the diagnostic steps to find the fault.
- P0325 Code Meaning: Knock Sensor 1 Circuit (Bank 1)P0325 means the ECM cannot read knock sensor 1. The engine retards timing defensively, causing power loss and poor fuel economy. Usually sensor or wiring.
- P0327 Code Meaning: Knock Sensor 1 Circuit Low (Bank 1)P0327 means the knock sensor 1 signal on Bank 1 is below the expected range. Learn symptoms, common causes, and the diagnostic path to fix it.
- P0335 Code Meaning: Crankshaft Position Sensor A CircuitP0335 means the ECU lost signal from the crankshaft position sensor. Common causes: failed sensor, damaged reluctor ring, or wiring faults.
- P0340 Code Meaning: Camshaft Position Sensor A CircuitP0340 means the camshaft position sensor A circuit (Bank 1) has failed or lost signal. Causes long crank times, stalling, and no-start conditions.
Emissions and EVAP
- P0401 Code Meaning: EGR Flow Insufficient DetectedP0401 means not enough exhaust gas is being recirculated. Learn symptoms, common causes like carbon buildup, and the diagnostic path.
- P0402 Code Meaning: EGR Flow Excessive DetectedP0402 means excessive exhaust gas recirculation flow detected. Learn the common causes, symptoms like rough idle and stalling, and the diagnostic path.
- P0403 Code Meaning: EGR Control Circuit MalfunctionP0403 means an electrical fault in the EGR control circuit—failed solenoid, broken wiring, or ECM driver issue. Symptoms, causes, and diagnostic steps.
- P0420 Code: Catalyst Efficiency Below Threshold (Bank 1)P0420 means your catalytic converter isn't cleaning exhaust efficiently. Learn what triggers this code, common causes, symptoms, and diagnostic steps.
- P0430 Code: Catalyst Efficiency Below Threshold (Bank 2)P0430 means Bank 2 catalytic converter efficiency is below threshold. Learn symptoms, common causes, and the diagnostic path before replacing parts.
- P0440 Code Meaning: EVAP System Malfunction ExplainedP0440 means EVAP system malfunction detected. Learn symptoms, common causes like gas cap and hoses, and the diagnostic path to fix it.
- P0441 Code: EVAP Incorrect Purge Flow Meaning & DiagnosisP0441 means purge flow doesn't match the commanded amount—not a leak. Learn the difference, common causes, and the diagnostic path.
- P0442 Code Meaning: EVAP System Leak Detected (Small Leak)P0442 means a small vapor leak in your EVAP system. Learn symptoms, common causes ranked by likelihood, and the diagnostic path to find it.
- P0443 Code Meaning: EVAP Purge Control Valve CircuitP0443 means the EVAP purge control valve circuit has a malfunction. Learn symptoms, causes, diagnosis steps, and repair options.
- P0446 Code Meaning: EVAP Vent Control Circuit MalfunctionP0446 means the EVAP vent valve circuit has failed. Learn symptoms, common causes ranked by likelihood, and the diagnostic path to fix it.
- P0449 Code Meaning: EVAP Vent Valve Circuit MalfunctionP0449 means the EVAP vent valve circuit has a fault. Learn symptoms, the most common causes, diagnostic steps, and repair options.
- P0455 Code Meaning: EVAP System Leak Detected (Gross Leak)P0455 means a large leak in your vehicle's evaporative emission system. Learn what causes it, why the gas cap is often to blame, and how to diagnose it.
- P0456 Code Meaning: EVAP System Very Small Leak DetectedP0456 means a very small leak in your EVAP system, often a gas cap or cracked hose. Learn symptoms, common causes, and the diagnostic path.
- P0457 Code Meaning: EVAP System Leak DetectedP0457 means evaporative emission system leak detected (fuel cap loose/off). Learn symptoms, causes, diagnosis steps, and repair options.
- P0496 Code Meaning: EVAP System High Purge FlowP0496 means the EVAP system is purging fuel vapor at a higher rate than commanded. Learn causes, symptoms, diagnosis steps, and repair options.
Sensors and electrical
- P0101 Code Meaning: MAF Sensor Range/Performance ProblemP0101 means the MAF sensor reading is implausible against other sensors. Learn symptoms, causes ranked by likelihood, and whether cleaning helps.
- P0102 Code Meaning: MAF Sensor Circuit Low InputP0102 means the mass air flow sensor reports less airflow than expected. Common causes: a dirty MAF element, a disconnected intake duct, a vacuum leak.
- P0106 Code Meaning: MAP Sensor Range/Performance ProblemP0106 means the manifold absolute pressure sensor reading is implausible. Learn symptoms, common causes, and the diagnostic path.
- P0107 Code Meaning: MAP Sensor Circuit Low InputP0107 means the manifold absolute pressure sensor signal is below the plausible range. Learn symptoms, common causes, and the diagnostic path.
- P0113 Code Meaning: Intake Air Temperature Sensor HighP0113 means the intake air temperature sensor signal is higher than plausible range. Learn symptoms, common causes, and the diagnostic path.
- P0121 Code Meaning: Throttle Position Sensor RangeP0121 means the throttle position sensor signal is implausible. Common causes: failed TPS, carbon buildup, wiring faults. May trigger limp mode.
- P0125 Code Meaning: Insufficient Coolant TemperatureP0125 means the engine didn't warm enough for closed-loop fuel control. Usually a stuck-open thermostat or faulty coolant temp sensor.
- P0128 Code Meaning: Coolant Temperature Below ThresholdP0128 means the engine didn't reach operating temperature in time. Usually a thermostat stuck open, bad coolant sensor, or low coolant.
- P0131 Code: O2 Sensor Low Voltage (Bank 1, Sensor 1)P0131 means the upstream oxygen sensor on Bank 1 is reading persistently low voltage. Learn whether it's a failed sensor or a real lean condition.
- P0133 Code: O2 Sensor Slow Response (Bank 1, Sensor 1)P0133 means the upstream oxygen sensor on Bank 1 switches too slowly between rich and lean. Learn causes, symptoms, and diagnostic steps.
- P0135 Code: O2 Sensor Heater Circuit (Bank 1, Sensor 1)P0135 means the upstream oxygen sensor heater on Bank 1 has failed. Learn symptoms, common causes, and the diagnostic path for this fuel-trim code.
- P0136 Code Meaning: O2 Sensor Circuit (Bank 1, Sensor 2)P0136 means the downstream oxygen sensor circuit on Bank 1 has malfunctioned. Learn symptoms, common causes, and the diagnostic path.
- P0138 Code: O2 Sensor High Voltage (Bank 1, Sensor 2)P0138 means the downstream oxygen sensor on Bank 1 is reading high voltage. Learn causes, symptoms, and the diagnostic path to fix it.
- P0140 Code Meaning: O2 Sensor No Activity (Bank 1, Sensor 2)P0140 means the downstream oxygen sensor on Bank 1 shows no signal activity. Usually a failed sensor, open wiring, or blown heater fuse.
- P0141 Code: O2 Sensor Heater Circuit (Bank 1, Sensor 2)P0141 means the downstream oxygen sensor heater circuit is out of range. Usually safe to drive short distances; affects emissions and fuel economy.
- P0521 Code Meaning: Oil Pressure Sensor Range/PerformanceP0521 means the oil pressure sensor signal is implausible. Check oil level and verify actual pressure with a gauge before assuming sensor failure.
- P0562 Code Meaning: System Voltage Low ExplainedP0562 means charging system voltage is below threshold. Learn whether it's your battery, alternator, or terminals—and the diagnostic path to fix it.
- P0563 Code: System Voltage High Meaning & DiagnosisP0563 means system voltage is too high, usually from a failed alternator voltage regulator. Symptoms, causes ranked by likelihood, and how to diagnose.
Transmission
- P0700 Code Meaning: Transmission Control System MIL RequestP0700 means the transmission control module found a fault and turned on the check engine light. What to do next, and why a basic scanner isn't enough.
- P0715 Code: Input Speed Sensor Circuit Meaning & DiagnosisP0715 means the TCM lost input speed sensor signal, causing erratic shifts or limp mode. Learn symptoms, common causes, and how to diagnose this code.
- P0730 Code Meaning: Incorrect Gear Ratio ExplainedP0730 means the transmission's actual gear ratio doesn't match what the computer commanded. Learn symptoms, common causes, and diagnostic steps.
- P0740 Code Meaning: Torque Converter Clutch CircuitP0740 means the torque converter clutch is not engaging as commanded. Learn symptoms, common causes, and the diagnostic path to fix this transmission code.
- P0741 Code Meaning: Torque Converter Clutch Stuck OffP0741 means the torque converter clutch is commanded to lock but does not engage. Learn symptoms, common causes, and the diagnostic path.
- P0755 Code Meaning: Shift Solenoid B Malfunction ExplainedP0755 means Shift Solenoid B has an electrical or performance fault. Learn symptoms, common causes, diagnostic steps, and whether it's a cheap fix.
Network (U-codes)
- U0100 Code Meaning: Lost Communication With ECM/PCM "A"U0100 means the network lost communication with the engine control module. Learn why multiple warning lights appear together and how to diagnose it.
- U0101 Code Meaning: Lost Communication With TCMU0101 means lost communication with the transmission control module. Learn symptoms, common CAN bus causes, and the diagnostic path to fix it.