GM Historic (OBD I) Diagnostics
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5
✓If CHECK ENGINE light is
functional, the Field Service
screen is displayed. Pressing
ENTER will toggle the short to
Pins A and B On and Off.
✓Continue with either the KOEO
procedure or KOER procedure.
✓While in Field Service mode, no new DTCs are stored in the
vehicle’s memory.
KOEO Procedure
✓In the KOEO Procedure, Field Service can check relays, solenoids
and the idle speed motor, and obtain DTCs using the CHECK
ENGINE light.
1) Turn ignition key on but do not start engine.
2) On Scan Tool, place Field Service On.
3) The CHECK ENGINE light begins to flash codes. Each DTC is
displayed three (3) times. The DTCs are displayed starting with
the lowest numbered one. After all DTCs are displayed, the
sequence keeps repeating until the ignition key or Field Service
mode is turned OFF.
❒Count the number of times the CHECK ENGINE light flashes to
obtain DTCs. The first digit is sent first. A short pause sepa-
rates digits while a long pause separates each DTC. All codes
contain 2 digits.
❒DTC 12 (No RPM reference pulse) should display first since
the engine is not running. If not, problems exist in the PCM or
CHECK ENGINE light circuitry.
Code 12 will look like:
FLASH-pause-FLASH-FLASH — long pause.
Code 23 will look like:
FLASH-FLASH-pause-FLASH-FLASH-FLASH — long pause.
Press ENTER to turn
Field Service On.
Field Service: Off
Field Service