P0300: Random or Multiple Cylinder Misfire
P0300 means the engine computer detected misfires that are not limited to one cylinder.
High if the check engine light is flashingGuide library
OBD-II Code Guides
Trouble codes are clues, not final diagnoses. Use them with symptoms and test results.
Symptom first
Safety check
Likely causes
Mechanic notes
P0171: System Too Lean Bank 1
P0171 means the engine computer is adding fuel because it sees too much air or not enough fuel on bank 1.
ModerateP0420: Catalyst System Efficiency Below Threshold
P0420 means the catalytic converter is not storing oxygen as expected, or the sensor data makes it appear that way.
Moderate unless drivability is poorP0455: Large EVAP System Leak Detected
P0455 means the evaporative emissions system cannot hold pressure or vacuum during a leak test.
Low to moderateP0128: Coolant Temperature Below Thermostat Regulating Temperature
P0128 usually means the engine is warming up too slowly or not reaching the expected operating temperature.
ModerateP0135: Oxygen Sensor Heater Circuit Bank 1 Sensor 1
P0135 means the upstream oxygen sensor heater circuit is not working as expected.
ModerateP0301: Cylinder 1 Misfire Detected
P0301 means the engine computer detected a misfire on cylinder 1.
High if the light is flashingP0302: Cylinder 2 Misfire Detected
P0302 means cylinder 2 is misfiring often enough for the computer to flag a fault.
High if the light is flashingP0303: Cylinder 3 Misfire Detected
P0303 indicates a misfire isolated to cylinder 3.
High if the check engine light flashesP0304: Cylinder 4 Misfire Detected
P0304 means the engine computer has detected misfire activity on cylinder 4.
High if the light flashesP0174: System Too Lean Bank 2
P0174 means the engine computer is adding extra fuel because bank 2 is running lean.
ModerateP0430: Catalyst System Efficiency Below Threshold Bank 2
P0430 means the catalytic converter monitor for bank 2 is below the expected efficiency threshold.
Moderate unless power is reducedP0440: EVAP System Malfunction
P0440 means the evaporative emissions system failed a general leak or flow test.
Low to moderateP0442: Small EVAP Leak Detected
P0442 means the EVAP system has a small leak, often too small to see without testing.
Low to moderateP0456: Very Small EVAP Leak Detected
P0456 points to a very small leak in the fuel vapor system.
LowP0101: Mass Airflow Sensor Range or Performance
P0101 means the MAF sensor reading is outside the range the computer expects.
ModerateP0113: Intake Air Temperature Sensor Circuit High
P0113 means the intake air temperature signal is higher than expected electrically, often indicating an open circuit.
Low to moderateP0118: Engine Coolant Temperature Sensor Circuit High
P0118 means the coolant temperature sensor circuit is reading out of range, often as extremely cold.
ModerateP0507: Idle Control System RPM Higher Than Expected
P0507 means idle speed is higher than the computer expects.
ModerateP0700: Transmission Control System Malfunction
P0700 means the transmission control module has requested the engine computer to turn on the warning light.
Moderate to highP0740: Torque Converter Clutch Circuit Malfunction
P0740 means the torque converter clutch control circuit or operation is not behaving as expected.
ModerateP0172: System Too Rich Bank 1
P0172 means the engine computer is removing fuel because bank 1 appears to be running too rich.
ModerateP0102: Mass Airflow Circuit Low Input
P0102 means the mass airflow sensor signal is lower than expected.
ModerateP0441: EVAP Incorrect Purge Flow
P0441 means purge flow through the EVAP system is not what the computer expected.
Low to moderateP0443: EVAP Purge Control Valve Circuit
P0443 means the electrical circuit for the purge control valve has a fault.
Low to moderateP0496: EVAP Flow During Non-Purge
P0496 means fuel vapor is flowing when the EVAP purge system should be closed.
Low to moderateP0325: Knock Sensor Circuit Malfunction
P0325 means the engine computer detected a problem in the knock sensor circuit.
ModerateP0520: Engine Oil Pressure Sensor Circuit
P0520 means the oil pressure sensor or circuit signal is not believable to the engine computer.
High if oil pressure warning is activeP0521: Engine Oil Pressure Sensor Range or Performance
P0521 means the oil pressure sensor signal is outside the expected range for engine conditions.
High if paired with oil warning or engine noiseP0401: Exhaust Gas Recirculation Flow Insufficient
P0401 means the EGR system is not flowing as much exhaust gas as expected.
ModerateP0402: Exhaust Gas Recirculation Flow Excessive
P0402 means the EGR system appears to be flowing too much exhaust gas.
ModerateP0404: EGR Circuit Range or Performance
P0404 means the EGR position or control signal is not matching what the computer expects.
ModerateP0505: Idle Control System Malfunction
P0505 means the engine computer cannot control idle speed as expected.
ModerateP0121: Throttle Position Sensor Range or Performance
P0121 means the throttle position signal does not match expected range or behavior.
Moderate to high if reduced power occursP0122: Throttle Position Sensor Circuit Low
P0122 means the throttle position sensor signal is lower than expected.
Moderate to high if drivability is poorP0201: Injector Circuit Cylinder 1
P0201 means the fuel injector circuit for cylinder 1 has an electrical fault.
High if misfiring badlyP0202: Injector Circuit Cylinder 2
P0202 means the fuel injector circuit for cylinder 2 has an electrical fault.
High if misfiring badlyP0203: Injector Circuit Cylinder 3
P0203 means the fuel injector circuit for cylinder 3 has an electrical fault.
High if misfiring badlyP0204: Injector Circuit Cylinder 4
P0204 means the fuel injector circuit for cylinder 4 has an electrical fault.
High if misfiring badlyP0130: Oxygen Sensor Circuit Bank 1 Sensor 1
P0130 means the upstream oxygen sensor circuit on bank 1 is not behaving as expected.
ModerateP0141: Oxygen Sensor Heater Circuit Bank 1 Sensor 2
P0141 means the downstream oxygen sensor heater circuit on bank 1 has a fault.
Low to moderateP0155: Oxygen Sensor Heater Circuit Bank 2 Sensor 1
P0155 means the upstream oxygen sensor heater circuit on bank 2 has a fault.
ModerateP0305: Cylinder 5 Misfire Detected
P0305 means the engine computer detected a misfire on cylinder 5.
High if the light is flashingP0306: Cylinder 6 Misfire Detected
P0306 means the engine computer detected a misfire on cylinder 6.
High if the light is flashingP0705: Transmission Range Sensor Circuit
P0705 means the transmission range sensor signal is missing, inconsistent, or out of expected range.
Moderate to high if the vehicle will not shift or start normally