Field Service Mode Check

NOTE: Field service mode check can only be performed on vehicles equipped with 12-pin DLC with a wire present in test terminal "B". See DATA LINK CONNECTOR (DLC) TEST TERMINALS table.

NOTE: Oxygen sensor may cool off while engine is idling. This causes system to go into "open loop". To restore "closed loop" mode, run engine at part throttle several minutes and accelerate from idle to part throttle several times.

Field service mode check confirms proper fuel system operation and verifies "closed loop" operation. Clear codes and perform this test after any repair is completed. When performing this check, always engage parking brake and block DRIVE wheels. Parking brake on FWD models does NOT hold drive wheels:

Start engine. With engine running, ground DLC terminal "B". See Fig 1 and Fig 2 . In "closed loop" mode, Malfunction Indicator Light (MIL) will flash once a second.
In "open loop", MIL will flash 2.5 times a second. If MIL is off most of the time, a lean exhaust is indicated. If MIL is on most of the time, a rich exhaust is indicated.

DATA LINK CONNECTOR (DLC) TEST TERMINALS

Application

Ground/Test

12-Pin DLC

3.1L (VIN M)

⁽¹⁾

3.1L (VIN T)

"A" & "B"

3.4L (VIN X)

⁽¹⁾

3.8L (VIN L) "W" Body

⁽¹⁾

16-Pin DLC

3.8L (VIN L)

⁽¹⁾

⁽¹⁾	Tech 1 scan tester required to perform On-Board Diagnostic (OBD) system check.

Fig 1: DLC Connector Terminal ID (1 Of 2)

Courtesy of GENERAL MOTORS CORP.

Fig 2: DLC Connector Terminal ID (2 Of 2)

Courtesy of GENERAL MOTORS CORP.