ENGINE CONTROLS 1F – 29
DAEWOO V–121 BL4
ON–BOARD DIAGNOSTIC (OBD II) SYSTEM CHECK
Circuit Description
The On–Board Diagnostic (OBD II) System Check is the
starting point for any driveability complaint diagnosis. Be-
fore using this procedure, perform a careful visual/physi-
cal check of the powertrain control module (PCM)/engine
control module (ECM) and the engine grounds for cleanli-
ness and tightness.
The OBD II System Check is an organized approach to
identifying a problem created by an electronic engine con-
trol system malfunction.
Diagnostic Aids
An intermittent may be caused by a poor connection,
rubbed–through wire insulation or a wire broken inside the
insulation. Check for poor connections or a damaged har-
ness. Inspect the PCM/ECM harness and connections for
improper mating, broken locks, improperly formed or dam-
aged terminals, poor terminal–to–wire connection, and
damaged harness.
Test Description
Numbers below refer to the step number on the Diagnostic
Chart:
1. The Malfunction Indicator Lamp (MIL) should be
ON steady with the ignition ON and the engine
OFF. If not, go to ”Diagnostic Aids”.
2. Checks the Class 2 data circuit and ensures that
the PCM/ECM is able to transmit serial data.
3. This test ensures that the PCM/ECM is capable of
controlling the MIL and the MIL driver circuit is not
shorted to ground.
4. If the engine will not start, refer to ”Engine Cranks
But Will Not Run” in this section.
7. A scan tool parameter which is not within the typical
range may help to isolate the area which is causing
the problem.
10. This vehicle is equipped with a PCM/ECM which
utilizes an electrically erasable programmable read
only memory (EEPROM). The replacement PCM/
ECM must be programmed. Refer to the latest
Techline procedure for PCM/ECM reprogramming.
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