On-Board Diagnostics: Notes

The Powertrain Control Module (PCM) constantly looks at the information from various sensors and other inputs and controls systems that affect vehicle performance and emissions. The PCM also performs diagnostic tests on various parts of the system. The PCM can recognize operational problems and alert the driver via the Malfunction Indicator Light (MIL). When the PCM detects a malfunction, the PCM stores a Diagnostic Trouble Code (DTC). The problem area is identified by the particular DTC that is set. The PCM supplies a buffered voltage to various sensors and switches. The input and output devices in the PCM include analog-to-digital converters, signal buffers, counters, and output drivers. The output drivers are electronic switches that complete a ground or voltage circuit when turned on. Most PCM controlled components are operated via output drivers. The PCM monitors these driver circuits for proper operation and, in most cases, can set a DTC corresponding to the controlled device if a problem is detected.

PCM is equipped with a self-diagnostic system which detects system failures or abnormalities. When a malfunction occurs, PCM will store a DTC and, if DTC is emissions related, the MIL is illuminated. The following are some of the functions that the PCM controls:

• The engine fueling.
• The Ignition Control Module (ICM).
• The Knock Sensor (KS) system.
• The Evaporative Emissions (EVAP) system.
• The Secondary Air Injection (AIR) system (if equipped).
• The Exhaust Gas Recirculation (EGR) system.
• The automatic transmission functions.
• The generator.
• The A/C clutch control.
• The cooling fan control.

There are 2 paths for accessing on-board diagnostics. It is necessary to access information through both paths, as each path presents some different information.

• Generic OBD-II  - This provides all generic codes and some OEM codes, serial data PIDs required for generic OBD-II and some OEM PIDs, monitor status, pending codes and freeze frame.
• OEM  - All DTCs, all PIDs, failure records, enhanced scan tool information and all other OEM scanner functions.