On-Board Diagnostics: Notes

NOTE: For more information on ECM operation, see THEORY & OPERATION - 2.2L ION article.

The ECM constantly looks at the information from various sensors and other inputs and controls systems that affect vehicle performance and emissions. The ECM also performs diagnostic tests on various parts of the system. The ECM can recognize operational problems and alert the driver via the Malfunction Indicator Light (MIL). When the ECM detects a malfunction, the ECM stores a Diagnostic Trouble Code (DTC). The problem area is identified by the particular DTC that is set. The ECM supplies a buffered voltage to various sensors and switches. The input and output devices in the ECM 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 ECM controlled components are operated via output drivers. The ECM 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.

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

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