Field Service Mode

In this mode, the CHECK ENGINE light will indicate if ECM is operating in open loop or closed loop.

Open loop operation is indicated when the CHECK ENGINE light flashes steadily at 2 1/2 times per second. Open loop operation exists when the O2 sensor signal is not being processed by ECM.

If CHECK ENGINE light flashes once per second, closed loop operation is indicated. CHECK ENGINE light will remain either off most of the time, indicating a lean condition, or on most of the time, indicating a rich condition. Closed loop operation exists when ECM is receiving and processing O2 sensor signal.

When system is in field service mode, new fault codes cannot be stored in ECM memory. When vehicle is in field service mode, it will go into closed loop when O2 sensor is at operating temperature and internal timer is by-passed. Internal timer switches vehicle to closed loop operation.

Code	Component	Circuit Fault
12	HEI Reference Signal	KOEO (Normal Condition)
13	O2 Sensor	Open Circuit
14	Coolant Temp. Sensor	High Temperature Signal
15	Coolant Temp. Sensor	Low Temperature Signal
21	Throttle Position Sensor	Signal Voltage High
22	Throttle Position Sensor	Signal Voltage Low
24	Vehicle Speed Sensor	Signal Fault
32	EGR System	System Failure
33	MAP Sensor	Signal Voltage High (Low Vacuum)
34	MAP Sensor	Signal Voltage Low (High Vacuum)
42	Elec. Spark Timing Circuit	Signal Fault
44	O2 Sensor Circuit	Lean Exhaust Signal
45	O2 Sensor Circuit	Rich Exhaust Signal
51	Faulty MEM-CAL	Circuit Fault
53	System Voltage	Overvoltage Signal