Monitoring Process

To achieve the requirements of the H025 Monitoring diagnosis, diagnostic tests have been designed to monitor the following:
- Heater voltage and current.
- Period of front HO2S reaction
- Other parameter (Tv) to monitor sensor aging.





Heater voltage and current
For quick and proper function of the oxygen sensor, the sensor element must be heated. A non functioning heater delays the sensor readiness for closed loop operation and thus influences emissions.
Therefore, the monitoring system measures sensor heater current (voltage drop over a shunt) and the heater voltage (heater supply voltage) and calculates the sensor-heater resistance to verify that the heater operation is normal. The monitoring function is activated once per trip (key cycle). Once the heater has been switched ON for a certain period of time, the current falls to a static value.

Period of front H02 reaction
The ECM is supplied with information regarding the oxygen content of the exhaust gases by the front HO2S. The signal is used to compute the injection period. The front HO2S monitors the contaminated exhaust gases. This, in time, causes aging of the device and a change in the signal.
To monitor the front HO2S responsiveness, the response period of the upstream oxygen sensor signal is compared with a lower and upper threshold. This comparison is done in an applicable range of engine speed and load with the engine at operating temperature. If the actual mean value of the pen d is outside of the lower and upper period thresholds the diagnostic trouble code is set.

Other parameter (Tv) to monitor sensor aging
The sensor aging is monitored by comparing the front and rear H02 and also using the correction value (Tv) which determines the mean value of

The sensor aging is monitored by comparing the front and rear H02 and also using the correction value (Tv) which determines the mean value of the lean/rich shift necessary for optima I air/fuel ratio-control. With the help of this information, the ECM can make allowance for aging of the front HO2S in its injection period calculations. When the parameter-Tv is above or below the specific limits, the ECM will set the diagnostic trouble code H025 Slow Response by aging.