Operation

Location:

Operation:

The upstream heated oxygen sensor is located at the catalytic converter inlet and detects the concentration of oxygen in the raw exhaust gases; an internal heater reduces the warm up time of the sensor output
An oxygen sensor (without heater) is located at the converter outlet

The ECM receives input signals from the sensors & varies the fuel injector duration (mixture) to provide optimum gas emissions.


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

If the downstream sensors indicate lean Air/Fuel Ratio (AFR) & the fuel system has judged that the fueling is rich, 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.

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.

Downstream sensor slow, high and no activity judgements are similar to upstream but the mapped values are different. If no activity is seen the AFR is enriched to force a response, and a failure judgement is made should this be unsuccessful.

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

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.


Circuit Diagram, O2 Sensors/ECM