NOTE: Oxygen (O2) 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. For field service mode check on PFI Cadillac vehicles, see appropriate G - TESTS W/ CODES article in this section.

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 test terminal "B" of the ALDL diagnostic connector. See Fig 1. In "closed loop" mode, SERVICE ENGINE SOON light will flash once a second.
In "open loop", light will flash 2.5 times a second. If light is off most of the time, a lean exhaust is indicated. If light is on most of the time, a rich exhaust is indicated.

Fig 1: Identifying ALDL Connector Terminals

Courtesy of GENERAL MOTORS CORP.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Field Service Mode Check (Except PFI CADILLAC)

Use the Manual With the Right Hardware

When to See a Mechanic

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