Powertrain Management: Description and Operation

ON BOARD DIAGNOSTIC SYSTEM CHECK
There is a continual self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in this section. The language for communicating the source of a malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the Powertrain Control Unit (PCM), a diagnostic trouble code is set and the Malfunction Indicator Lamp (MIL) "Service Engine Soon/Check Engine" is illuminated on some applications.

MALFUNCTION INDICATOR LAMP (MIL)
The MIL (Service Engine Soon/Check Engine) lamp is on the instrument panel and has the following functions:

^ It informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as reasonably possible.

^ Some applications will flash diagnostic trouble codes stored by the PCM which help the technician diagnose system problems.

^ Some applications will flash "Open Loop" or "Closed Loop" operation.

Refer to Testing and Inspection / Vehicle On-Board Diagnostics / Without Scan Tool for information on identifying exactly what information the MIL can provide. Without Scan Tool

As a bulb and system check, the MIL (Service Engine Soon/Check Engine) 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 but a diagnostic trouble code will remain stored.

When the light remains "ON" while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, an On-Board Diagnostic System Check must be performed. Testing and Inspection
These checks will expose malfunctions which may not be detected if other diagnostics are performed prematurely.