Post-Catalyst Heated Oxygen Sensor Monitoring

Requirements
The post-catalyst oxygen sensor diagnostic is designed to monitor:

^ How much the sensor output voltage changes.
^ How long it takes sensor voltage to change by a given amount. The sensor will be considered to be malfunctioning when it's output voltage, response rate or other parameters reach a level which cause the vehicle to exceed 1.5 times the FTP emissions standards for that vehicle.

Theory of Diagnostic Operation
Once the enabling criteria for the post-catalyst heated oxygen sensor have been met, the ECM begins to monitor sensor output If the catalyst is doing it's job properly the rate of change in the output from the rear sensor will be fairly slow when compared to the front sensor. The test for the post- catalyst sensor will run once per trip. There is no second or third chance to pass the test as was the case with the pre-catalyst sensor.

Once all the enabling criteria have been fulfilled a sensor will pass the diagnostic test IF:

1. Sensor voltage rises once to above 400 mV within 500 seconds.
AND
2. Sensor voltage changes by more than 500 mV within 500 seconds.

If either one of these requirements has not been met during each of two consecutive driving cycles, the MIL will be turned on and a DTC set.