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GM Historic (OBD I) Diagnostics
5
Field Service
Mode can be operated with the ignition Key On-Engine Off
(KOEO) or with the Key On-Engine Running (KOER).
✓ Vehicles equipped with climate control computers do not use Field Ser-
vice Mode.
✓ Some 1994 & 1995 vehicles equipped with a 12-pin ALDL connector with
pins A and B shorted will not cause the CHECK ENGINE light to flash
codes.
Select
Field Service
from the GM
Function List and press
.
Turn ignition Key On, but do not start
engine. The Check Engine light should
flash. Select YES to continue. If the light
does not illuminate, either the vehicle
does not support this test mode, the
lamp is burned-out or circuit problems
exist.
✓ It is difficult to know exactly which GM vehicles use the Field Service
mode. It is possible to have the Field Service mode function on the Func-
tion List, and it not be applicable.
If Check Engine light operates, the Field
Service screen is displayed. Pressing
will toggle the short to Pins A &
B On and Off.
Continue with either the KOEO procedure or KOER procedure. When done,
press
to return to the GM Function List.
✓ While in
Field Service
mode, no new trouble codes are stored in the vehi-
cle’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 will begin 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 CHECK ENGINE Light Flashes to obtain trouble codes. The first
digit is sent first. A short pause separates digits while a long pause
separates each DTC. All codes contain 2 digits.