Heated Oxygen Sensor Diagnostic

Heated Oxygen Sensor Diagnostic

Heated oxygen sensors are used in the Bosch system. These sensors are checked as usual for short-circuits and open-circuits. When these faults occur, corresponding errors are stored for each sensor.

Function of oxygen sensors
The basic functionality of these sensors is the concept of a pump current of the oxygen in fuel-air mixtures, from where the sensor system can compute the actual fuel-air mixture. Heating of the sensors is also undertaken in order to decrease the internal resistance and to further improve the performance of the sensors.

The internal resistance of the front sensors is constantly monitored for the heating diagnosis. A comparison is therefore made with a reference in order to consider aging and sample deviations. Also, power stage diagnosis is made for the front sensors where a comparison of the control signal (input) and the output signal is made. Through this procedure all various possible types of short-circuits will be detected.

Diagnosis of oxygen sensors
Diagnosis of incorrect lambda measurements due to shunting effects is performed through a lambda offset of the downstream-control if a difference of air-fuel mixture exceeds a threshold (3%). By monitoring the voltage output of the specific processor CJ125 a check is made that it operates correct, avoiding hardware errors. Insufficient heating of the LSU, i.e., the front sensor, and disconnection of the pump current are detected through a comparison of the fuel-air mixture with the rear sensors. The criterion is that if the front sensor indicates a fuel-air mixture with a ratio of 1, while the rear sensor indicates a lean or rich mixture, one of these failures has occurred. Through different comparisons of the front and rear sensors, also short-circuits and high resistance to battery and ground, are detected for the front sensors. Low resistance connection between heater and the sensor, i.e. the heater coupling, is detected by monitoring lambda changes due to the heater pulse rate. A decrease of the actual performance, known as the dynamics, of the sensor due to aging or fouling can be detected through a comparison of the estimated (model based) signal and the actual measured signal.

Similar testing as these above are also undertaken for the rear sensors, where the major differences in the diagnosis can be found through oscillation checks, checking of the sensor voltage and the dynamics during fuel cut-off, for the rear sensors.

During active oxygen sensor aging diagnosis the sensor signal (shape and frequency) can be considered as characteristic for the quality of the installed upstream sensor. Thus, for this purpose several parameters are calculated continuously. These calculated values are then provided via a tester interface (Scan Tool), together with the correction value of the downstream controller, the dynamic property value of the upstream continuous sensor and different constants by this tester interface. It is by this functionality the legislative authorities (for instance CARB) determine the standard of the oxygen sensor system.