Field Service Mode
In this mode, the SERVICE ENGINE SOON light will indicate if the ECM is operating in open loop or closed loop.
Open loop operation is indicated when the SERVICE ENGINE SOON light flashes steadily at 2.5 times per second. Open loop operation exists when the O2 sensor signal is not being processed by the ECM.
Closed loop operation is indicated by a SERVICE ENGINE SOON light that flashes once per second. The SERVICE ENGINE SOON light will stay off most of the time (indicating a lean condition) or on most of the time (indicating a rich condition). Closed loop operation exists when the ECM is receiving and processing the O2 sensor signal.
When the system is in field service mode, new fault codes cannot be stored in the ECM memory. When the vehicle is in field service mode, it will go into closed loop when the O2 sensor is at operating temperature and the internal timer is by-passed. Internal timer switches the vehicle to closed loop operation.
| Code | Component | Circuit Fault |
|---|---|---|
| 12 | HEI Reference Signal | KOEO (Normal Condition) |
| 13 | Oxygen 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 |
| 23 | Manifold Air Temp. Sensor | Temperature Too Low |
| 24 | Vehicle Speed Sensor | Signal Fault |
| 25 | MAT Sensor | Temperature Too High |
| 32 | EGR System | System Error |
| 33 | MAP Sensor | Signal Voltage High (Low Vacuum) |
| 34 | MAP Sensor | Signal Voltage Low (High Vacuum) |
| 42 | Elec. Spark Timing Circuit | Signal Fault |
| 44 | Oxygen Sensor Circuit | Lean Exhaust Signal |
| 45 | Oxygen Sensor Circuit | Rich Exhaust Signal |
| 51 | Faulty MEM-CAL | Circuit Fault |
| 53 | System Voltage | Overvoltage Signal |