TEST SEQUENCE
In many instances, emissions systems must fail diagnostic tests more than once before the Powertrain Control Module (PCM) illuminates the MIL. These tests are known as two trip monitors. Other tests that illuminate the MIL after a single failure are known as one trip monitors. A trip is defined as: start the vehicle and operate it to meet the criteria necessary to run the given monitor.
Many of the diagnostic tests must be performed under certain operating conditions. However, there are times when tests cannot be run because another test is in progress (conflict), another test has failed (pending) or the Task Manager has set a fault that may cause a failure of the test (suspend).
DIAGNOSTIC TROUBLE CODES (DTCS)
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.
Priorities
- Priority 1 One-Trip Failure of Non-Fuel or Non-Mis-Fire Fault (e.g., Cat Mon Failure)
- Priority 3 Matured Fault (either One-Trip or Two-Trip) Non-Fuel and Non-Mis-Fire
- Priority 4 One-Trip Failure of Fuel System or Mis-Fire Fault
- Priority 6 Matured Fault for Fuel System or Mis-Fire (either One-Trip or Two-Trip)
Non-emissions 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.
DTC SELF ERASURE
With one trip components or systems, the MIL is illuminated upon test failure and DTCs are stored.
Two trip monitors are components requiring failure in two consecutive trips for MIL illumination. Upon failure of the first test, the Task Manager enters a maturing code. If the component fails the test for a second time the code matures and a DTC is set.
After three good trips the MIL is extinguished and the Task Manager automatically switches the trip counter to a warm-up cycle counter. DTCs are automatically erased following 40 warm-up cycles if the component does not fail again.
For misfire and fuel system monitors, the component must pass the test under a Similar Conditions Window in order to record a good trip. A Similar Conditions Window is when engine RPM is within ±375 RPM and load is within ±20% of when the fault occurred.
Note. It is important to understand that a component does not have to fail under a similar window of operation to mature. It must pass the test under a Similar Conditions Window when it failed to record a Good Trip for DTC erasure for misfire and fuel system monitors.
DTCs can be erased anytime with a scan tool. Erasing the DTC with the scan tool erases all OBD II information. The scan tool automatically displays a warning that erasing the DTC also erases all OBD II monitor data. This includes all counter information for warm-up cycles, trips and freeze frame data.