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.