Diagnostic Trouble Codes and Freeze Frames

Diagnostic Trouble Codes and Freeze Frames

The ECM and automatic TCM software monitors each fault condition and allocates a mnemonic (or memory) 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 greater than or equal to 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.