Monitoring Procedure (Sensor)

Monitoring Procedure
These diagnostics are closely linked to the fuel system diagnostics.

The sensor outputs are monitored during steady driving with a fully warm engine.

If the downstream sensors indicate lean Air / Fuel Ratio (AFR) and the fuel system has judged fuel system rich (P0172 or P0175) then the upstream sensor is judged to have failed high voltage.

The upstream sensor is judged to have failed low voltage by comparison of its output with that of the downstream sensor.

The upstream slow response judgements are made when any of the following switching rates remain above a threshold: low to high switch time from one mid point of the switching cycle to the next mid point.

If the switching rate of the upstream sensors falls below a value mapped against load and speed then the sensor is judged to have failed low activity.

Downstream sensor slow response and no activity judgements are similar to upstream but the mapped values are different. Also if no activity is seen the AFR is enriched to force a response, and a failure judgement is only made after this has been unsuccessful.

Downstream high and low voltage judgements are made after the sensor has remained above or below predetermined thresholds for a long period.

Both upstream and downstream heater circuit judgements are made by comparing the expected heater drive state with the actual state. If these states are different for too long then the heater circuits are judged to be faulty.

For all the above diagnostics, the relevant DTC is stored if the failure judgement is made on two successive trips.

Fig. 1 H02S Sectional Diagram:

Fig. 2 H02S With Connector:

Structure of Heated Oxygen Sensor
The heated oxygen sensor detects the concentration of oxygen in the exhaust gases. In operation, the ECM receives input signals from the sensor and varies the injector opening time duration. The sensor has an internal heater to stabilize the sensor output.

Relevant Diagnostic Trouble Codes

Bank A Upstream
P0131
P0132
P0133
P0134
P0135

Bank A Downstream
P0137
P0138
P0139
P0140
P0141

Bank B Upstream
P0151
P0152
P0153
P0154
P0155

Bank B Downstream
P0157
P0158
P0159
P0160
P0161