On-Board Diagnostic System

The Powertrain Control Module (PCM) performs self-diagnostics of powertrain control functions. The PCM communicates faults with a system of diagnostic trouble codes (DTCs). These DTCs may or may not turn on the MIL (SERVICE ENGINE SOON) or SERVICE lamp to alert the driver.

The malfunction indicator lamp MIL (SERVICE ENGINE SOON) telltale lamp is in the instrument cluster and it informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as reasonably possible.


As a bulb and system check, the lamp will come On with the key On and the engine not running. When the engine is started, the lamp will turn Off. If the lamp remains On, the self-diagnostic system has detected a problem. If the problem goes away, the lamp will go out in most cases after three trips, but a diagnostic trouble code will remain stored in the PCM.

If the lamp remains On while the engine is running, or when a malfunction is suspected due to a driveability problem an on-board diagnostic system check (OBD system check) must be performed. These checks will expose malfunctions which may not be detected if other diagnostics are performed prematurely. Refer to the On Board Diagnostic System Check [OBD System Check]. Testing and Inspection


The diagnostic trouble tree charts and functional checks are designed to locate a faulty circuit or component through logic based on the process of elimination. The charts are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are no multiple failures. Trouble tree charts are designed to be used only when a diagnostic trouble code is active.

Diagnosis of the system would begin with the "On-Board Diagnostic (OBD) System Check" which is the starting point for communication with the Saturn's On-Board Diagnostic System. Testing and Inspection