General Information

DIAGNOSTIC INFORMATION
The control module performs a continual self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in the DIAGNOSTIC CHARTS

The control module's ability for displaying the source of a malfunction in the system is called Diagnostic Trouble Codes [DTC(s)]. The Diagnostic Trouble Codes [DTC(s)l are two digit numbers that can range from 12 to 82. When a malfunction is detected by the control module, a Diagnostic Trouble Code (DTC) will set, and the Malfunction Indicator Lamp (MIL) "Service Engine Soon" is illuminated.
The diagnostic trouble code charts and functional checks in Diagnostic Charts 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 or without multiple failures.

MALFUNCTION INDICATOR LAMP (MIL) "SERVICE ENGINE SOON"
The Malfunction Indicator Lamp (MIL) "Service Engine Soon" is in the instrument panel and has the following functions:
^ It informs the driver that a fault has occurred, and the vehicle should be taken for service as soon as reasonably possible.
^ It displays DTC stored by the control module, helping the technician diagnose system faults.
^ It indicates "Open Loop" or "Closed Loop" operation.
As a bulb and system check, the lamp will come "ON" with the ignition "ON" and the engine not operating. When the engine is started, the lamp will go "OFF." If the light remains "ON," the self- diagnostic system has detected a problem. If the fault goes away, the light will go out in most cases after 10 seconds, but a DTC will remain stored in the control module memory.
When the lamp remains "ON" while the engine is operating, or when a malfunction is suspected due to a driveability or emissions problem, the DIAGNOSTIC SYSTEM CHECK must be performed. Testing and Inspection
The procedures for these checks are given in DIAGNOSTIC CHARTS.

These checks can expose malfunctions which may not be detected if other diagnostics are performed prematurely.

INTERMITTENT MALFUNCTION INDICATOR LAMP (MIL) "SERVICE ENGINE SOON"
In the case of an "Intermittent" problem, the MIL will light for ten (10) seconds and then will go out. However, the corresponding Diagnostic Trouble Code (DTC) will be stored in the memory of the control module unless the battery voltage to the control module has been removed. When unexpected DTC(s) appear during the code displaying process, assume that these DTC(s) were set by an intermittent malfunction and can be helpful in diagnosing the system.
An intermittent DTC may or may not reset.

NOTE: If it is an intermittent failure, a Diagnostic Trouble Code Chart will not be used.

Consult the "Diagnostic Aids" found with the text of the diagnostic trouble code chart corresponding to the intermittent code. DIAGNOSIS BY SYMPTOM also covers the topic of INTERMITTENTS.
Testing and Inspection
A physical inspection of the applicable sub-system most often will resolve the fault.

DATA LINK CONNECTOR (DLC)
The twelve terminal Data Link Connector (DLC) is wired to the control module and is located under the left side of the instrument panel. This connector has terminals that diagnose the system by means of a jumper wire or a TECH 1 scan tool.

BACK-UP FUEL (Also Called Back-Up or 3.9K)
In this Control Module Information mode, the control module is operating on the fuel back-up logic used to control the fuel delivery if the control module fails. This mode verifies that the back-up feature is OK. The parameters that can be read on a scan tool in this mode are not of much use for service.