Downstream Sensor Diagnosis Phase

If the catalyst diagnosis has completed without any problem, the downstream sensor is treated as «good» and a sensor diagnosis is not necessary.

If a problem has occurred with the downstream sensor during the catalyst diagnosis, a sensor diagnosis is done.

If throughout the catalyst diagnosis phase, repeated several times, the downstream sensor has not reacted, the A/F closed loop mode is delayed in order to test the sensor.

If the downstream sensor signal indicates a rich (lean) mixture, the injection time is forced to lean (rich) until the filtered downstream sensor signal switches over or until the end of a delay. If this delay expires, the sensor is treated as failed. This may be a result of:

• A leak in the exhaust line,
• A damaged sensor.

Electrical failures (short circuit and open circuit of signal and heater) are detected during the «comprehensive components» diagnostics.