Heated Oxygen Sensors Diagnostic

Heated Oxygen Sensors Diagnostic

Two heated oxygen sensors (HO2S) are used in the Denso system. The heated oxygen sensors are checked as usual for short-circuits and open-circuits. If either of these faults occurs, (front heated oxygen sensor failure) or (rear heated oxygen sensor failure) is stored. In addition, the signal from the front heated oxygen sensor is checked. It can cause emission increase if its response time decreases too much or if the mean A/F ratio in closed lop is too far from the catalyst efficiency window. This check is carried out as follows: the ECM receives exhaust composition data from the front heated oxygen sensor. Using this information the engine control module calculates injection period. Because the front heated oxygen sensor is located upstream of the TWC it is subject to attack by the hot uncleaned exhaust gases. This leads to the sensor ageing and its signal changing. These changes are monitored by checking:
- Front heated oxygen sensor mean signal voltage
- Rear heated oxygen sensor control
- Front heated oxygen sensor period.

To determine the front heated oxygen sensor's mean voltage, the engine control module uses the rear heated oxygen sensor signal. This signal provides information about the exhaust gas oxygen composition downstream of the catalytic converter. The engine control module can use this information to take account of front sensor ageing in it's injection period computations first injection period is calculated on the basis of the front heated oxygen sensor signal, then the rear heated oxygen sensor mean voltage is checked. A defective sensor may also have the correct mean signal voltage. For this reason the front sensor response time is also monitored. The ECM monitors how long it takes for the front heated oxygen sensor to switch from lean to rich to lean again. When the response time becomes too long for a number of diagnostic tests, a DTC for front heated oxygen sensor failure is stored.

The rear heated oxygen sensor response time is also checked. Fuel trim is carried out in a controlled cycle and if a signal from the rear heated oxygen sensor is not received within a set time a DTC for the rear heated oxygen sensor failure is stored. Twin heated oxygen sensor (HO2S) control starts and remains active when:
- The front heated oxygen sensor has started control
- The rear heated oxygen sensor has reached operating temperature
- Engine coolant temperature exceeds a limit temperature
- Mass air flow is between two target values
- Engine speed is between two target values

Twin heated oxygen sensor control- is suspended during fuel shut-off, if the engine control module detects misfiring or if the engine control module detects a fault in:
- Three-way catalytic converter
- Heated oxygen sensor preheating
- Mass air flow sensor
- Engine coolant temperature sensor.