When the Check Engine Light (CEL) comes on, the vehicle’s computer stores an OBD-II Diagnostic Trouble Code (DTC). Most codes begin with a letter followed by four numbers, such as P0300 or P0420.
What the First Letter Means
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P = Powertrain — engine, transmission, fuel, and emissions
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B = Body — airbags, climate control, seats, lighting, etc.
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C = Chassis — ABS, steering, suspension, and related systems
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U = Network/Communication — communication problems between vehicle modules
For a Check Engine Light, P-codes are the most common.
Common Check Engine Light Codes
| Code | Meaning | Common Causes |
|---|---|---|
| P0100 | Mass or Volume Air Flow Circuit Malfunction | Bad MAF sensor, wiring problem, intake leak |
| P0101 | MAF Sensor Range/Performance Problem | Dirty MAF sensor, air leak, clogged air filter |
| P0113 | Intake Air Temperature Sensor Circuit High | Bad IAT sensor, disconnected connector, damaged wiring |
| P0128 | Coolant Temperature Below Thermostat Regulating Temperature | Stuck-open thermostat, coolant temperature sensor |
| P0130 | O2 Sensor Circuit Malfunction Bank 1 Sensor 1 | Bad oxygen sensor, wiring issue, exhaust leak |
| P0133 | O2 Sensor Slow Response Bank 1 Sensor 1 | Aging O2 sensor, fuel mixture problem |
| P0135 | O2 Sensor Heater Circuit Malfunction | Failed O2 sensor heater, fuse, wiring |
| P0171 | System Too Lean Bank 1 | Vacuum leak, dirty MAF sensor, weak fuel pressure |
| P0172 | System Too Rich Bank 1 | Leaking injector, excessive fuel pressure, MAF problem |
| P0174 | System Too Lean Bank 2 | Vacuum leak, MAF sensor, fuel delivery problem |
| P0201–P0208 | Fuel Injector Circuit Malfunction | Injector failure, wiring, PCM driver problem |
| P0300 | Random/Multiple Cylinder Misfire Detected | Spark plugs, ignition coils, injectors, compression, vacuum leak |
| P0301 | Cylinder 1 Misfire | Plug, coil, injector, compression issue on cylinder 1 |
| P0302 | Cylinder 2 Misfire | Plug, coil, injector, compression issue on cylinder 2 |
| P0303 | Cylinder 3 Misfire | Plug, coil, injector, compression issue on cylinder 3 |
| P0304 | Cylinder 4 Misfire | Plug, coil, injector, compression issue on cylinder 4 |
| P0325 | Knock Sensor Circuit Malfunction | Bad knock sensor, damaged wiring |
| P0335 | Crankshaft Position Sensor Circuit Malfunction | Failed crankshaft sensor, wiring, reluctor problem |
| P0340 | Camshaft Position Sensor Circuit Malfunction | Cam sensor, wiring, timing problem |
| P0401 | EGR Flow Insufficient | Clogged EGR passages, bad EGR valve, sensor problem |
| P0402 | EGR Flow Excessive | Stuck-open EGR valve, control system fault |
| P0420 | Catalyst System Efficiency Below Threshold Bank 1 | Worn catalytic converter, O2 sensor, exhaust leak |
| P0430 | Catalyst System Efficiency Below Threshold Bank 2 | Catalytic converter, O2 sensor, exhaust leak |
| P0440 | EVAP System Malfunction | Gas cap, EVAP leak, purge or vent valve |
| P0441 | Incorrect EVAP Purge Flow | Purge valve, vacuum hose, EVAP system problem |
| P0442 | Small EVAP Leak Detected | Loose gas cap, cracked EVAP hose, small leak |
| P0446 | EVAP Vent Control Circuit Malfunction | Vent valve, wiring, blocked vent |
| P0455 | Large EVAP Leak Detected | Loose/missing gas cap, disconnected EVAP hose |
| P0456 | Very Small EVAP Leak Detected | Gas cap seal, small hose leak, EVAP valve |
| P0500 | Vehicle Speed Sensor Malfunction | Speed sensor, wiring, tone ring |
| P0505 | Idle Control System Malfunction | Throttle body, idle control valve, vacuum leak |
| P0562 | System Voltage Low | Weak battery, alternator, charging-system problem |
| P0700 | Transmission Control System Malfunction | Transmission module has stored additional codes |
| P0715 | Input/Turbine Speed Sensor Circuit Malfunction | Transmission speed sensor, wiring |
| P0720 | Output Speed Sensor Circuit Malfunction | Output speed sensor, connector, wiring |
Understanding Misfire Codes
Codes P0301 through P0312 usually identify the cylinder experiencing a misfire. For example:
P0301 = Cylinder 1 misfire
P0302 = Cylinder 2 misfire
P0304 = Cylinder 4 misfire
P0306 = Cylinder 6 misfire
Common causes include worn spark plugs, failed ignition coils, faulty injectors, vacuum leaks, low compression, or incorrect fuel pressure.
What to Do When the Check Engine Light Comes On
If the light is solid, the vehicle may usually still be driven cautiously, but the code should be diagnosed soon.
If the Check Engine Light is flashing, there may be a severe misfire that can quickly damage the catalytic converter. Reduce driving and diagnose the vehicle as soon as possible.
Connect an OBD-II scanner, read all stored and pending codes, record the freeze-frame data, inspect the related system, repair the actual fault, clear the codes, and road-test the vehicle.
Important: A trouble code identifies the system where a problem was detected. It does not automatically mean the component named in the code needs to be replaced. For example, P0420 does not always mean the catalytic converter itself is bad; exhaust leaks, misfires, fuel-mixture problems, or oxygen-sensor issues can also contribute.