Field Service Mode Check (Fuel Injected Models)
On fuel injected models, "SERVICE ENGINE SOON" light will indicate operational mode of engine if ALDL is grounded while engine is running. In "closed loop" mode, "SERVICE ENGINE SOON" light will flash at a rate of one flash per second. In "open loop", light will flash at a rate of 2.5 flashes per 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.
This test confirms proper operation of fuel system 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.
NOTE:
On some engines, oxygen sensor will cool off after only a short period of time while engine is idling. This will cause engine 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.