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Field Service Mode Check (Fuel Injected)

NOTE: On some engines, oxygen sensor will 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.

This 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.

  1. Start engine. With engine running, ground "test" terminal "B" of the ALDL diagnostic connector. See Figure . In "closed loop" mode, SERVICE ENGINE SOON light will flash once a second.
  2. 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.
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