Oxygen Sensor: Description and Operation

COMPONENTS

HO2 (Oxygen) sensor
Two heated O2 Sensors are used in the vehicle. The sensors produce voltages from 0 to 1 volt, depending upon the oxygen content of the exhaust has in the exhaust manifold. When a large amount of oxygen is present (caused by a lean air/fuel mixture), the sensors produce a low voltage. When there is a lesser amount (rich air/fuel mixture) the sensors produce a higher voltage. By monitoring the oxygen content and converting it to electrical voltage, the sensors act as a rich-lean switch.

Both HO2 Sensors are equipped with a heating element that keeps the sensors at proper operating temperature during all modes of operation. Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation faster and allows the system to remain in closed loop during periods of extended idle.

Front HO2 sensor
The front (upstream) HO2 Sensor is located in the exhaust downpipe after the TWC and provides an input voltage to the ECM. The input tells the ECM the oxygen content of the exhaust gas. The ECM uses this information to fine tune the air/fuel ratio by adjusting injector pulse width.

Rear HO2 sensor
The rear (downstream) HO2 Sensor is located near the outlet end of the catalytic convertor. The rear HO2 Sensor input is used to detect catalytic convertor deterioration. As the convertor deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the rear HO2 Sensor input to the input from the front sensor, the ECM calculates catalytic convertor efficiency.




MONITORING

System description
Effective control of exhaust emission is achieved by an oxygen "feedback system" to the ECM. This "feedback" is accomplished with the HO2 Sensors. Once the HO2 Sensor reaches operating temperature, the sensor generates a voltage signal to the ECM that is inversely proportional to the amount of oxygen in the exhaust. This voltage signal is used by the ECM to calculate the fuel injector pulse width to maintain the ideal 14.7 to 1 Air Fuel (A/F) ratio. This is the ideal mixture because at this ratio, the catalyst works best to remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) from the exhaust.

Monitoring process
To achieve the requirements of the HO2S 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
The voltage readings taken from the HO2 Sensor are very temperature sensitive and are not particularly accurate below 300 °C. For this reason, heating of the HO2 sensor is necessary to allow the engine controller to shift to closed loop control as soon as possible. The heating element used to heat the HO2 Sensor must be tested to ensure that it is heating the sensor properly.

- Period of front HO2 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 period 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 HO2S and also using the correction value (Tv) which determines the mean value of the lean/rich shift necessary for optimal 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 HO2S Slow Response by aging.

MIL illumination
If any fault relating to HO2S is detected, then the MIL is only turned ON if the same fault is again detected during a second drive cycle.