Fault Criteria, Front Heated Oxygen Sensor

The oxygen sensor's ability to react quickly to changes in the fuel-air mixture is diagnosed once per trip.
The sensor's transition frequency is measured when the conditions for diagnosis are fulfilled.
Transition is what happens when the sensor voltage goes from high to low or from low to high.
If the transition frequency is too low, the sensor is defective.

The number of sensor transitions taking place during 265 engine revolutions is counted.
If the number exceeds 12 the diagnosis is OK
If the number of transitions is less than 12, two additional attempts to obtain a "diagnosis OK" result this trip will be made.
If the sensor is failed on all three diagnostic occasions the fault criteria are satisfied.

In other respects the sensor and its preheater are monitored according to the On-Board Diagnostic I model.

Conditions For Diagnosis
^ Diagnosis not yet performed this trip
^ Vehicle speed = 0
^ Idling speed
^ Closed loop activated
^ Coolant temperature above 60 °C (140 °F)
^ Sensor preheater current less than 1,500 mA
^ No load changes