Got a check engine light? Our car fault code guides explain every OBD2 trouble code in plain English — what it means, what causes it, and how to fix it. Browse by code or use the categories below to find the fault you’re dealing with.
Most Common OBD2 Fault Codes
| Code | Description | Common Cause | Severity |
|---|---|---|---|
| P0420 | Catalyst System Efficiency Below Threshold (Bank 1) | Faulty catalytic converter or O2 sensor | Medium |
| P0300 | Random/Multiple Cylinder Misfire Detected | Bad spark plugs, ignition coils, fuel injectors | High |
| P0171 | System Too Lean (Bank 1) | Vacuum leak, MAF sensor, fuel pressure | Medium |
| P0128 | Coolant Temp Below Thermostat Regulating Temp | Faulty thermostat | Low |
| P0442 | Evaporative Emission System Leak Detected (Small) | Loose or faulty fuel cap, EVAP purge valve | Low |
| P0101 | Mass Air Flow Sensor Range/Performance | Dirty or faulty MAF sensor | Medium |
| P0113 | Intake Air Temperature Sensor High Input | Faulty IAT sensor or wiring | Low |
| P0401 | Exhaust Gas Recirculation Flow Insufficient | Blocked or faulty EGR valve | Medium |
Browse Codes by Category
- P0xxx — Powertrain (Engine & Transmission) — The most common codes. Covers fuel system, ignition, emissions, and transmission
- P1xxx — Manufacturer-Specific Powertrain — Brand-specific codes that vary by make (e.g., Ford, Toyota, BMW)
- B0xxx — Body Codes — Airbag, windows, central locking, comfort systems
- C0xxx — Chassis Codes — ABS, traction control, suspension systems
- U0xxx — Network Codes — CAN bus communication failures between modules
How to Read OBD2 Fault Codes
To read fault codes, you need an OBD2 scanner. You can use a Bluetooth OBD2 adapter with a free app, or a dedicated handheld scanner. Simply plug into the OBD2 port (usually under the dashboard on the driver’s side), connect, and read codes.
Should You Clear Fault Codes Without Fixing the Problem?
Clearing a fault code without fixing the underlying issue will just cause the light to come back — usually within a few drive cycles. Always diagnose and fix the root cause first. Clearing codes is useful after a repair to confirm the fix was successful and to reset readiness monitors before an MOT/emissions test.
Featured Fault Code Guides
- P0420 — Catalyst System Efficiency Below Threshold
- P0300 — Random Cylinder Misfire
- How to Fix a Check Engine Light
- How to Read OBD2 Codes (Step-by-Step)
More fault code guides coming soon. Contact us if you’d like us to cover a specific code.