Component Tests and General Diagnostics

Diagnosis of Downstream Catalyst Lambda Sensor

This diagnostic function detects all electrical connection faults of the lambda sensor.

The diagnostic function is performed if the temperature of the downstream catalyst exceeds the dew-point for more than a given period. This means the sensor heating of the downstream catalyst is turned on for this time with active lambda control, if the exhaust gas temperature of the downstream catalyst is below a certain threshold and the battery voltage exceeds a certain threshold.

If the sensor voltage of the downstream catalyst remains within the plausible voltage range for longer than a given time, then a wiring interruption or defective sensor heating is assumed.

If the internal resistance Ri of the sensor exceeds a certain threshold while the modeled catalyst temperature is greater than a certain threshold, then a wire interruption of sensor ground is assumed.

If the voltage of the sensor of the downstream catalyst is continuously above the maximum value threshold for longer than a given period, then a short circuit of the sensor signal wire to battery voltage is assumed.

If the sensor voltage of the downstream catalyst lies below the threshold without interruption for longer than a given period (with active lambda control, switched off secondary air and secondary air diagnosis, no secondary air system and canister purge system failure), then a wire-to-wire short circuit between sensor signal and ground lead is assumed.

Diagnosis of Lambda Sensor Downstream Catalyst







Aging Monitor of the Rear Lambda Sensor

Oscillation Check
The function checks whether the sensor output voltage of the rear lambda sensor (Nernst type) always remains above or below a specified threshold.

If the rear closed loop lambda control, which uses the sensor signal of the rear lambda sensor, is active the lambda sensor voltage must cross a threshold at least once within a specified period of time (~ 65 seconds).

If the sensor voltage remains above or below the threshold, a test function will be started, the system switches to rich or lean operation (Delta lambda(rich) = 10 % , Delta lambda(lean) = 7%). If the voltage was above the threshold, there will be a lean operation and vice versa. This function is interrupted by fuel cut-off, by very low load conditions, etc. If the sensor voltage reacts appropriately (crossing the threshold in the expected direction), the sensor is o.k. If there is no reaction, the sensor is judged as faulty, an error flag will be set and the rear closed loop control will be switched off.

Aging Monitoring of Rear Lambda Sensor - Oscillation Check