Diagnostic Trouble Code Priority

With OBD-II, different DTC faults have different priorities according to regulations. As a result, the priorities determine MIL illumination and DTC erasure. DTCs are entered according to individual priority. DTCs with a higher priority overwrite lower priority DTCs.

1. Priority 0  - This is a non-emissions related DTC.
2. Priority 1  - One trip failure of a two trip DTC for non-fuel system and non-misfire conditions.
3. Priority 2  - One trip failure of a two trip DTC for fuel system rich or fuel system lean condition, or misfire condition.
4. Priority 3  - Two trip failure for non-fuel system or non-misfire condition, or a matured one trip comprehensive component fault.
5. Priority 4  - Two trip failure for matured fuel system rich or fuel system lean and misfire condition, or a one trip catalyst damaging misfire.

Non-emission related failures have no priority. One trip failures of two trip faults have low priority. Two trip failures or matured faults have higher priority. One and two trip failures of fuel system and misfire monitor take precedence over non-fuel system and non-misfire failures.