DIAGNOSTIC TROUBLE CODES & FREEZE FRAMES
The ECM and automatic TCM software monitors each fault condition and allocates a mnemonic Diagnostic Trouble Code (DTC) to specific faults; e.g. P0170 fuel trim malfunction. The software also checks that the monitoring conditions are valid and the current status of the fault. There are common condition flags for each fault module.
Generally, an emission relevant fault is not reported as soon as it occurs, but only after it is flagged during a second valid drive cycle. A drive cycle is defined by a period of engine operation equal to, or more than 10 seconds and the diagnostic fault path in question having been completed at least once. If the fault is still present on the subsequent drive cycle, the OBD system logs the fault and freeze frame data and illuminates the MIL.
If the fault is not present in the subsequent driving cycle, the system holds it as a temporary fault and counts a number of drive cycles before deleting it from the fault memory providing it does not reoccur. A re-occurring fault will be immediately logged as a permanent emissions fault, and may illuminate the MIL according to the type of fault.
When an emissions fault is recognized, the system monitors over Warm Up Cycles (WUC). A warm up cycle is defined by a period of engine operation where the ECT has increased by 21°C (40°F) and exceeds 71°C (160°F).
Monitoring during warm up is also relevant to permanent faults. If the flagged fault is not present in a subsequent drive cycle, the warm up cycle counter is started. If the fault is not flagged again, the MIL remains illuminated but is extinguished after 3 fault free WUC. The fault is finally deleted from the fault memory after 40 fault free WUC.
In the case of misfire monitoring two levels of misfire are checked
- Emission relevant misfire is monitored over 1000 engine revolutions and 2 drive cycles.
- Catalyst damage misfire is monitored over 200 engine revolutions. If the threshold is exceeded in any 200 engine revolutions segment the MIL is immediately flashed to signal the driver to reduce engine load. When the misfire decreases below the catalyst damage threshold or ceases altogether the MIL is permanently illuminated.
If the freeze frame memory is free the first occurring fault will store freeze frame data regardless of the source. If a subsequent fault occurs, the current freeze frame data is not overwritten unless this fault is of higher freeze frame priority. CARB faults, freeze frame data and other parameters can be read through the diagnostic port via a generic scan tool.
Thermostat Diagnosis
For additional thermostat diagnosis J1979 Mode $06 Data testing see scheme 6
- DATA 3 (m6cthm) - Bit 0 - 6: Number of the measuring path within the TID = CID. Bit 7: Type of test limit: 0 = Test Limit Is Maximum Value - Test fails if test value > test limit. 1 = Test Limit Is Minimum Value - Test fails if test value < test limit.
- DATA 4 + 5 (m6wthm) - 2-byte value of the measured value.
- DATA 6 + 7 (m6sthm) - 2-byte value of the threshold value.
Passive Secondary Air Injection Diagnostic
For this test to run the front O2 sensors must have been ready for operation for longer than a certain time, the secondary air injection system must be operating, the engine speed and load must be within a pre-determined window, engine airflow must be less than an altitude dependent threshold and the ECT must be greater than a threshold. (Scheme 81)
The front O2 sensors are monitored over a time period and the minimum voltage value recorded. When a second timer expires, a test counter is incremented and the minimum sensor value is compared with a threshold. If the voltage is less than the threshold then a counter of good test results is incremented. When the test counter reaches a threshold, the number of good test results is compared with a limit value. If the number of good results is greater than the limit then the Secondary Air Injection system is functioning correctly, otherwise a fault is stored and the MIL is illuminated on the next drive cycle, if the fault is again present. see scheme 20
Scheme 80
Active Secondary Air Injection Diagnostic
If on any drive cycle during which secondary air injection operation has occurred, there are insufficient passive diagnostic test results for fault determination. The system will then attempt to perform an active check of the secondary air injection system. For an active test to occur, the vehicle must be at rest with the engine idling, feedback fuel control enabled, below an altitude threshold, with the engine having been running for longer than a pre-determined time and secondary air injection not operating. If the EVAP canister purge is operating, then it will be ramped down to zero.
The active test is in two parts. (Scheme 82) First the current feedback correction factor is recorded and the secondary air injection pump turned on, but with the control valves shut. If the fuelling enriches by more than a threshold, then the valves are leaking or stuck open, but if after a timer has elapsed the feedback correction is below the threshold, then the system proceeds with a flow check.
For the second part of the active diagnostic the valves are opened and if after a time limit, the feedback has not enriched the fuelling by more than a second threshold, then a problem exists with the system and if it is present again on a subsequent drive cycle, a fault is stored and the MIL illuminated. see scheme 20
Scheme 81
Scheme 82
Diagnosis of Fuel Delivery System
Faults in the fuel delivery system can occur which cannot be compensated for by the adaptive control. In this case the adaptive values leave a predetermined range. If the adaptive value is outside this predetermined range, and then if the condition is again present on a subsequent drive cycle, the MIL is illuminated and the appropriate diagnostic trouble codes are stored.