OBD2 check engine light codes: full list and meanings
When the check engine light comes on, the car's computer stores a code. An OBD2 scanner can read it, for example P0300 or P0420. The common codes are explained below.
Updated 12-Nov-2026
These are generic codes starting with P0. Under the international standard they mean the same on every car with OBD2. Codes starting with P1 are each maker's own and are not listed here. A code only shows where the computer saw a problem; a mechanic's tests show which part is at fault. Older carburettor cars don't have OBD2.
For any code, try this first
- If the check engine light is flashing, drive gently straight to a mechanic, or stop somewhere safe and call one.
- If the temperature needle is in the red, the red oil light comes on, or the engine makes a loud noise or smokes, stop somewhere safe straight away and switch off.
- Write down every code the scanner shows, not just this one. Do it before clearing them, because the other codes help the mechanic find the real cause faster.
- If the light came on after filling up, check that the fuel cap is closed until it clicks.
- With the engine off, check the battery terminals: they should be tight, with no white or green corrosion on them.
Safety: never remove the radiator cap on a hot engine; boiling coolant can burn you. Keep your hands away from the fan and belts while the engine runs. If you smell fuel, switch off, keep any flame or cigarette away and call a mechanic.
Air, temperature and throttle sensors (P0100 to P0128)
These sensors tell the computer how much air enters the engine, how hot things are and how far the accelerator is pressed.
| Code | Meaning |
|---|---|
| P0100 | Mass air flow (MAF) sensor: circuit fault. |
| P0101 | Mass air flow (MAF) sensor: reading outside the expected range (range/performance). Details |
| P0102 | Mass air flow (MAF) sensor: circuit low input. |
| P0103 | Mass air flow (MAF) sensor: circuit high input. |
| P0105 | Manifold absolute / barometric pressure (MAP) sensor: circuit fault. |
| P0106 | Manifold absolute / barometric pressure (MAP) sensor: reading outside the expected range (range/performance). |
| P0107 | Manifold absolute / barometric pressure (MAP) sensor: circuit low input. |
| P0108 | Manifold absolute / barometric pressure (MAP) sensor: circuit high input. |
| P0110 | Intake air temperature (IAT) sensor: circuit fault. |
| P0111 | Intake air temperature (IAT) sensor: reading outside the expected range (range/performance). |
| P0112 | Intake air temperature (IAT) sensor: circuit low input. |
| P0113 | Intake air temperature (IAT) sensor: circuit high input. |
| P0115 | Engine coolant temperature (ECT) sensor: circuit fault. |
| P0116 | Engine coolant temperature (ECT) sensor: reading outside the expected range (range/performance). |
| P0117 | Engine coolant temperature (ECT) sensor: circuit low input. |
| P0118 | Engine coolant temperature (ECT) sensor: circuit high input. |
| P0120 | Throttle / pedal position sensor 'A' (TPS): circuit fault. |
| P0121 | Throttle / pedal position sensor 'A' (TPS): reading outside the expected range (range/performance). |
| P0122 | Throttle / pedal position sensor 'A' (TPS): circuit low input. |
| P0123 | Throttle / pedal position sensor 'A' (TPS): circuit high input. |
| P0128 | Coolant thermostat: coolant temperature below the thermostat's regulating temperature. Details |
Oxygen (O2) sensors (P0130 to P0141)
Sensor 1 sits before the catalytic converter and Sensor 2 after it. Bank 1 is the side of the engine with cylinder 1.
| Code | Meaning |
|---|---|
| P0130 | Oxygen (O2) sensor, Bank 1 Sensor 1 (before the catalytic converter): circuit fault. |
| P0131 | Oxygen (O2) sensor, Bank 1 Sensor 1 (before the catalytic converter): circuit low voltage. |
| P0132 | Oxygen (O2) sensor, Bank 1 Sensor 1 (before the catalytic converter): circuit high voltage. |
| P0133 | Oxygen (O2) sensor, Bank 1 Sensor 1 (before the catalytic converter): slow response. |
| P0134 | Oxygen (O2) sensor, Bank 1 Sensor 1 (before the catalytic converter): no activity detected. |
| P0135 | Oxygen (O2) sensor, Bank 1 Sensor 1 (before the catalytic converter): heater circuit fault. Details |
| P0136 | Oxygen (O2) sensor, Bank 1 Sensor 2 (after the catalytic converter): circuit fault. |
| P0137 | Oxygen (O2) sensor, Bank 1 Sensor 2 (after the catalytic converter): circuit low voltage. |
| P0138 | Oxygen (O2) sensor, Bank 1 Sensor 2 (after the catalytic converter): circuit high voltage. |
| P0139 | Oxygen (O2) sensor, Bank 1 Sensor 2 (after the catalytic converter): slow response. |
| P0140 | Oxygen (O2) sensor, Bank 1 Sensor 2 (after the catalytic converter): no activity detected. |
| P0141 | Oxygen (O2) sensor, Bank 1 Sensor 2 (after the catalytic converter): heater circuit fault. |
Fuel and air mixture (lean / rich)
Bank 2 only exists on engines such as a V6 or V8. A 4-cylinder engine only has Bank 1.
Misfire, knock and crank/cam sensors (P0300 to P0340)
If the light flashes with a misfire code, don't drive fast and get to a mechanic soon.
| Code | Meaning |
|---|---|
| P0300 | Random or multiple cylinder misfire detected. Details |
| P0301 | Cylinder 1 misfire detected. Details |
| P0302 | Cylinder 2 misfire detected. Details |
| P0303 | Cylinder 3 misfire detected. Details |
| P0304 | Cylinder 4 misfire detected. Details |
| P0325 | Knock sensor 1 circuit fault (Bank 1 or single sensor). |
| P0328 | Knock sensor 1 circuit high input (Bank 1 or single sensor). |
| P0335 | Crankshaft position sensor 'A' circuit fault. Details |
| P0340 | Camshaft position sensor 'A' circuit fault. Details |
Exhaust and fuel vapour: EGR, catalytic converter, EVAP
EVAP codes are often caused by a loose fuel cap. Check the cap first.
| Code | Meaning |
|---|---|
| P0400 | Exhaust gas recirculation (EGR) flow fault. |
| P0401 | Exhaust gas recirculation (EGR) flow insufficient detected. |
| P0420 | Catalyst system efficiency below threshold (Bank 1). Details |
| P0430 | Catalyst system efficiency below threshold (Bank 2). |
| P0440 | Evaporative emission (EVAP) system fault. |
| P0442 | Evaporative emission (EVAP) system leak detected (small leak). Details |
| P0446 | Evaporative emission (EVAP) system vent control circuit fault. |
| P0455 | Evaporative emission (EVAP) system leak detected (large leak). Details |
| P0456 | Evaporative emission (EVAP) system leak detected (very small leak). |
Speed, idle and system voltage (P0500 to P0563)
For voltage codes, check the battery terminals and the battery's condition first.
| Code | Meaning |
|---|---|
| P0500 | Vehicle speed sensor 'A' fault. |
| P0505 | Idle air control system fault. Details |
| P0506 | Idle air control system RPM lower than expected. |
| P0507 | Idle air control system RPM higher than expected. |
| P0562 | System voltage low. |
| P0563 | System voltage high. |
Common questions
Why does the check engine light come on?
When the car's computer sees a fault in the engine, its sensors or the emission system, it turns the light on and stores a code. The cause can be as small as a loose fuel cap or as serious as a misfire, so it's important to read the code.
What should I do if the check engine light is flashing?
A flashing light usually means a severe misfire. Don't drive fast or load the car. Drive gently straight to a mechanic, or stop somewhere safe and call one. Carrying on can damage the catalytic converter and the engine.
How do I read the code with an OBD2 scanner?
The OBD2 socket is usually under the dashboard on the driver's side, below the steering wheel. With the car off, plug in the scanner, turn the key on and read the codes. Cheap Bluetooth scanners can also show codes through a phone app.
Does clearing the code fix the problem?
No. Clearing a code only turns the light off; the fault is still there. If the cause hasn't been fixed, the code will come back within minutes or days. Always write the code down before clearing it.
What do Bank 1, Bank 2, Sensor 1 and Sensor 2 mean?
Bank 1 is the side of the engine with cylinder 1. Bank 2 only exists on engines such as a V6 or V8; a 4-cylinder engine only has Bank 1. Sensor 1 is the O2 sensor before the catalytic converter, and Sensor 2 the one after it.
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