Heated Oxygen Sensor Diagnostics

Two heated oxygen sensors (HO2S) are used in the Motronic 4.4 engine control module. The heated oxygen sensors are checked as usual for short-circuits and open-circuits. If either of these faults occurs, diagnostic trouble code EFI-212 (front heated oxygen sensor) or EFI-153 (rear heated oxygen sensor) are stored. In addition, the signal from the front heated oxygen sensor is checked. This check is carried out as follows.

The engine control module receives exhaust composition data from the front heated oxygen sensor. Using this information the engine control module calculates injection period. Because the front sensor is located upstream of the three-way catalytic converter (TWC) it is subject to attack by the 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 front heated oxygen sensor 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 its injection period computations.

The mean rear sensor signal voltage should be approx. 0.6 V when twin heated oxygen sensor control is active. First the injection period is calculated on the basis of the front sensor signal. Then the mean rear sensor signal is calculated. If it is not 0.6 V the injection period calculations are adjusted so that the correct mean rear sensor signal will be achieved. When this adjustment exceeds a certain limit for a number of diagnostic tests, diagnostic trouble code EFI-436, rear heated oxygen sensor compensation, is stored.

When the engine is running the adjustment carried out by the rear heated oxygen sensor should be positive and negative, if not diagnostic trouble code EFI-425, rear heated oxygen sensor, control, will be stored.

Twin heated oxygen sensor control starts and remains active when:

^ The front heated oxygen sensor has started control.
^ The rear heated oxygen sensor has reached operating temperature.
^ Catalytic convener temperature exceeds +300°C (+572°F) (calculated by the engine control module).
^ Engine speed (RPM) is between 1,200-3,400 rpm.
^ Load is 1.3-5.0 ms (at 1,000 rpm), 1.34.0 ms (at 2,000 rpm), 1.3-3.0 ms (at 3,000 rpm).

Twin heated oxygen sensor control is suspended during fuel shut-off, if the engine control module detects misfiring (diagnostic trouble codes efi-45x, 543, 545, 55x) or if the engine control module detects a fault in:

^ The EVAP system (diagnostic trouble code EFI-315).
^ Three-way catalytic converter (diagnostic trouble code EFI-443).
^ Heated oxygen sensor preheating (diagnostic trouble code EFI-521 or EFI-522).
^ Mass air flow (MAF) sensor (diagnostic trouble code EFI-121).

A defective sensor may also have the correct mean signal voltage. For this reason the front sensor period is also monitored. The engine control module monitors how long it takes for the front heated oxygen sensor to switch from rich to lean to rich again. This is carried out at idling when the difference between a good and bad sensor is greatest. When the period becomes too long for a number of diagnostic tests, diagnostic trouble code EFI-435 Front heated oxygen sensor slow response, is stored.