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 |