OBD-II Trouble Codes Lookup & Guide
Learn what your car’s Check Engine Light means and find explanations for common OBD-II codes.
🔍 OBD-II Trouble Code Lookup Tool
Enter your OBD-II code (e.g., P0171, P0420) to see what it means.
What Are OBD-II Codes?
OBD-II (On-Board Diagnostics II) codes are standardized diagnostic trouble codes (DTCs) used in most cars and trucks manufactured after 1996. These codes help identify problems with your vehicle’s engine, transmission, emissions system, and other essential components.
Every time your Check Engine Light (CEL) turns on, your car’s onboard computer stores a specific OBD-II code that points to the source of the issue. Mechanics and DIYers use these codes to quickly find and fix problems.
How to Read OBD-II Codes
OBD-II codes typically follow the format Letter + Four Numbers, for example: P0301, C1234, B0020, or U0100.
| Code Part | Meaning |
|---|---|
| First Letter | System: P = Powertrain, B = Body, C = Chassis, U = Network |
| First Digit | 0 = Generic (SAE Standard), 1 = Manufacturer-specific |
| Last Three Digits | Specific fault or malfunction area |
Common OBD-II Codes
- P0171 — System Too Lean (Bank 1)
- P0300 — Random/Multiple Cylinder Misfire Detected
- P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
- P0440 — Evaporative Emission Control System Malfunction
- P0500 — Vehicle Speed Sensor Malfunction
How to Retrieve OBD-II Codes
- Plug an OBD-II scan tool into your vehicle’s diagnostic port (under the dashboard).
- Turn on the ignition (engine off).
- Read and note the codes displayed on the scanner.
- Search for the meanings using the lookup tool above.
What to Do After You Get a Code
- Write down all codes — there may be more than one.
- Look up each meaning using the tool above or your repair manual.
- Check simple causes first — loose gas cap, dirty sensor, unplugged wire.
- If uncertain, consult a certified mechanic.
Why OBD-II Codes Matter
- Explains why your Check Engine Light is on
- Helps you avoid unnecessary repairs
- Speeds up troubleshooting
- Improves understanding of vehicle performance
Disclaimer
This information is for general reference only. Always verify the exact fault using a certified mechanic or your vehicle’s service manual. Incorrect interpretation of OBD-II codes can lead to unnecessary repairs.