On-Board Diagnostics: Notes

This system has complete self-diagnostic capabilities. This systems uses a single Powertrain Control Module (PCM) that receives input from engine monitoring sensors. The PCM uses these input signals to control air/fuel mixture for emission control, fuel economy and good driveability.

Signals from various sensors are monitored continuously. If certain signals are lost or become faulty, PCM will adopt fixed substitute values (limp-home mode) to enable vehicle to be driven for certain failures. Substitute values can be adopted for Engine Coolant Temperature (ECT) sensor signal, Throttle Position (TP) sensor signal, Mass Airflow (MAF) sensor signal, and oxygen sensor (HO2S) signal.

Faults are recorded in PCM memory in the form of Diagnostic Trouble Codes (DTCs). Codes can be displayed using a generic scan tester connected to On-Board Diagnostics II (OBD-II) Data Link Connector (DLC).

PCM stores Diagnostic Trouble Codes (DTCs) differently in accordance with a predetermined pattern. An emission related fault must be present for a predetermined number of intervals before PCM permanently stores a DTC and illuminates MIL. If fault is cleared before a specified interval has passed, DTC is erased from memory. There are 3 main types of intervals:

1. Trip
   
   All diagnostic functions have been performed.
2. Running Cycle
   
   Fuel trim (closed loop) has been initiated.
3. Time
   
   Time in seconds.