OBD-II code reader - use and limitations
An OBD-II code reader unlocks diagnostic data from the vehicle. Used correctly it explains why the check-engine light is on, helps you plan workshop visits and prevents knock-on damage. For the dashboard context, read Gauges and warning lights .
What is OBD-II?
| Item | Explanation |
|---|---|
| Standard | Mandatory on petrol cars from 2001 and diesel cars from 2004 |
| Connector | 16-pin diagnostic plug, usually under the dashboard |
| Data | Diagnostic trouble codes (DTC), live sensor data and freeze frames |
OBD-II codes contain one letter plus four digits. The prefix shows the system:
| Prefix | System | Examples |
|---|---|---|
| P0xxx | Powertrain | Misfire, lambda sensor, pressure sensor |
| C0xxx | Chassis | ABS wheel speed, steering angle |
| B0xxx | Body | Airbag, HVAC |
| U0xxx | Network | CAN communication, module fault |
When should you read codes?
- When the check-engine light stays on or flashes
- After a repair to confirm the fault is resolved
- Before buying a used car to spot stored faults
- Ahead of the roadworthiness test to check emissions-related codes
Troubleshooting workflow
- Connect the reader with the ignition off.
- Switch the ignition on and pair via cable, Bluetooth or Wi-Fi.
- Read active and stored codes and note freeze-frame data.
- Look up the codes in manufacturer resources or trusted databases.
- Inspect wiring and sensors before clearing any codes.
- Test-drive and verify that the lamp and codes do not return.
For engine warnings, see Engine lights and indicators .
Limits and pitfalls
- Budget readers may only show generic P0 codes.
- Many apps cannot access manufacturer-specific modules or live data.
- Clearing codes without solving the cause can hide serious issues.
- Workshops can detect cleared codes through histories stored in control units.
Legal and practical notes
- Do not erase codes to hide faults before a vehicle sale; that can be illegal.
- Sharing vehicle data with cloud services requires consent, see Dashcam and privacy .
- Keep your own documentation of diagnostics if you rely on factory warranty coverage.