Heated Oxygen Sensor Diagnostics

HEATED OXYGEN SENSOR DIAGNOSTICS

Motronic 4.3 SFI uses two Heated Oxygen Sensor (HO2S) units on certain markets. These HO2S units can be checked as normal for signs of short circuiting and wiring breaks. If either of these faults occurs, DTC 2-1-2 (front HO2S) or DTC 1-5-3 (rear HO2S) will be recorded, and a check will be made on the correctness of the front HO2S signal as described briefly below.

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. Since the front HO2S is installed upstream of the TWO, it monitors the contaminated exhaust gases. This, in time, causes ageing of the device and a change in the signal. These phenomena are monitored in two ways:

^ By checking the mean value of the front HO2S signal
^ Checking the rear HO2S control
^ By checking the period of the front HO2S signal

The ECM uses the signal from the rear HO2S, which measures the oxygen content of the gases downstream of the TWO, to measure the mean value of the front HO2S signal. Using this information, the ECM can make allowance for ageing of the front HO2S in its injection computations.

The mean signal voltage from the rear HO2S should be approx. 0.6 V when two-sensor control is active. The injection period is first computed on the basis of the front HO2S signal and the mean value of the rear HO2S signal is then checked. If the value deviates from 0.6 V, the computed injection period is adjusted until the correct mean value is achieved. DTC 4-3-6 'Rear HO2S compensation' is recorded when the correction exceeds a given level during a certain number of successive checks.

While driving, modulation of the rear HO2S is to be both positive and negative; if not, DTC 4-2-5 Rear HO2S, modulation, will be set.

Two-HO2S modulation starts and is active when:

^ the front HO2S begins modulation and is active;
^ the rear HO2S has reached working temperature;
^ the TWO temperature exceeds +300°C (+5720°F) (as calculated by the ECM);
^ the engine speed is between 1200 and 3400 rpm;
^ the internal load is between 1.3-5.0 ms (at 1000 rpm), 1.3-4.0 ms (at 2000 rpm) and 1.3-3.0 ms (at 3000 rpm).

Two-sensor control is interrupted in the event of fuel cut-off, if the ECM detects misfiring (DTCs 4-5-X, 5-4-2, 5-4-3, 5-4-4, 5-4-5 and 5-5-X) or if the ECM detects a fault in any of the following:

^ EVAP system (DTC 3-1-5)
^ TWO (DTC 4-4-3)
^ HO2S heating (DTC 5-2-1 or 5-2-2)
^ MAF sensor (DTC 1-2-1)

Since even a defective HO2S may deliver the correct mean signal voltage, the period of the front HO2S signal is also monitored, the ECM measuring the time between successive changes of the signal from rich to lean This check is carried out at idling, when the difference between a satisfactory and poor sensor is greatest. DTC 4-3-5 'Front HO2S, slow response' will be recorded if the period remains too long during a series of successive checks.