For fuel injected engines, a special diagnostic mode is provided to help in oxygen sensor diagnosis. If the diagnostic terminal is grounded with the engine RUNNING, the system will enter field service mode. In this mode the MIL will display whether the engine control system is running in Open Loop or Closed Loop. When running in Closed Loop, the MIL will also display whether the engine is running lean or rich.
In Open Loop operation (during engine and oxygen sensor warm up) the MIL light will flash 2.5 times per second. Once the oxygen sensor is fully warmed and the control module decides that its feedback is reliable, the system will enter Closed Loop operation and the MIL will flash 1 time per second. During Closed Loop operation, if the MIL remains OFF most of the time, then the engine is running lean. If the MIL remains ON most of the time, the engine is running rich.