Pre-Catalyst Heated Oxygen Sensor Monitoring

Requirements
The diagnostic for the pre-catalyst oxygen sensor is designed to monitor the number of times the oxygen sensor makes the rich/lean or lean/rich transition in a given period of time.

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 enable criteria for the pre-catalyst oxygen sensor diagnostic have been met, the ECM will begin to monitor sensor output. A healthy oxygen sensor in a properly functioning fuel system will constantly switch back and forth from rich (sensor voltage approaching 1) to lean (sensor voltage approaching 0).

As a sensor deteriorates it will take longer and longer to make these transitions. In the 900 TRIONIC system, a sensor is considered good if there are more than 12 transitions per 265 engine revolutions. The oxygen sensor only needs to pass this test once per trip. If, for some reason, the number of transitions is too low on the first test, the sensor will be retested a second and, if needed, a third time before a failure is recorded. The MIL will not be turned on unless diagnosis is failed in two consecutive driving cycles.

Pre-Catalyst Oxygen Sensor Enabling Criteria
The pre-catalyst oxygen sensor diagnostic will run when:

^ The diagnostic test has not been completed yet this trip.
^ Vehicle speed is 0.
^ The engine is idling.
^ The fuel system is operating in closed loop.
^ Coolant temperature is above 140 F (60 C)
^ Oxygen sensor heater current is below 1500 mA.
^ There are no load changes (A/C, cooling fan, etc. should not be cycling)