How to Read and Clear OBD Fault Codes, A Workshop Guide
Blog › Diagnostics · By George Josefsson ·The check-engine light is the most common reason a car ends up in your workshop. Behind that light is a diagnostic trouble code (DTC), a standardized five-character code that tells you what triggered it. Here's how to read, interpret, and clear DTCs efficiently.
What is a DTC?
A DTC follows the format P0XXX (powertrain), C0XXX (chassis), B0XXX (body), or U0XXX (network). The first digit after the letter indicates whether it's a generic (0) or manufacturer-specific (1) code.
| Prefix | System | Examples |
|---|---|---|
| P0xxx | Powertrain (engine, transmission) | P0300 random misfire, P0171 lean mixture |
| P1xxx | Manufacturer-specific powertrain | Varies by make |
| C0xxx | Chassis (ABS, stability) | C0035 left front wheel speed sensor |
| B0xxx | Body (airbags, climate, lighting) | B0001 driver airbag circuit |
| U0xxx | Network (CAN bus communication) | U0100 lost communication with ECM |
Reading codes with MechMind
Connect a compatible BLE adapter to the OBD-II port, open MechMind in Chrome or Edge, and tap Connect. The app reads all stored DTCs automatically and displays them with plain-language descriptions.
No cable, no dedicated scan tool, no software to install. Just your browser.
When to clear, when to investigate
Clearing a DTC turns off the check-engine light and resets the freeze-frame data. That's fine for intermittent codes that don't return. But if you clear a code and the light comes back within a drive cycle, the fault is still active, clearing it again won't help.
Best practice:
- Read all stored codes and note them
- Check freeze-frame data (RPM, coolant temp, speed at the time of the fault)
- Fix the root cause
- Clear codes
- Test-drive and verify the light stays off
Common DTCs every workshop sees
| Code | Description | Usual cause |
|---|---|---|
| P0300 | Random/multiple cylinder misfire | Spark plugs, coil packs, fuel injectors |
| P0171 / P0174 | System too lean (bank 1 / bank 2) | Vacuum leak, weak fuel pump, dirty MAF sensor |
| P0420 | Catalyst efficiency below threshold | Worn catalytic converter, O2 sensor |
| P0128 | Coolant thermostat below regulating temp | Stuck-open thermostat |
| P0442 | EVAP system small leak | Loose or cracked fuel cap |
Pending vs. stored codes
Pending codes are faults detected once but not yet confirmed. They don't trigger the check-engine light. If the fault occurs again on the next drive cycle, the code moves to stored status and the light comes on.
Always check pending codes too, they can reveal developing issues before the customer notices.
Next steps
Need to know if your adapter supports the car on the lift? Check which vehicles work with OBD. For help picking the right adapter, see our adapter guide.
Try MechMind free, read your first DTC in under two minutes.