761 Oxygen Sensor

The pre-catalyst oxygen sensor is an important element in the control system that regulates the mixture strength (air-fuel ratio) in spark ignition engines. As explained in Chapter 5, the oxygen sensor samples the exhaust gas before it enters the catalytic converter and produces a signal that tells the ECM the air-fuel ratio of the mixture that is entering the combustion chambers. It is a feedback system. The oxygen sensor signal is used by the ECM to change the amount of fuel injected so that lambda is kept in the range of approximately 0.97-1.03. Figure 7.6 should remind you of the principle.

Fig. 7.6 Oxygen sensor - feedback principle

Should the oxygen sensor be disconnected, there will be no feedback to the ECM, which will probably have been programmed to use a substitute value to cause the 'limp home' mode to operate and to minimize the possibility of damage to the catalyst. Add to this the fact that, like the catalyst, the oxygen sensor needs to be at an operating temperature of over 300°C for it to operate efficiently, and it becomes evident that electrical testing of the oxygen sensor is work that requires care and attention to detail.

If we now take an example of an oxygen sensor fault code we can explore some of the details that should be taken into account when attempting diagnosis and rectification work.

On a particular vehicle, the fault code 51 means a fault at the 'oxygen sensor or in the oxygen sensor circuit'. Assuming that there are no other faults recorded that may affect the performance of the oxygen sensor, an in situ voltage test with the oscilloscope should assist in locating the problem. It is advisable to remember that the ECM has registered a fault code because the value received at the ECM is not within the programmed limits. It may be the case that the sensor is working correctly but the signal is not reaching the ECM because of a defect between the sensor output terminals and the connecting pins at the ECM. This indicates that the oxygen sensor should be checked:

1 at the ECM diagnostic port, using equipment such as the Bosch KTS 500 tester. This will produce a trace of the oxygen sensor voltage, similar to the one shown in Fig. 7.7. This particular trace was taken at idle speed and the frequency is quite low.

2 by backprobing at the sensor, as shown in Fig. 7.8.

Fig. 7.7 Voltage test at the ECM diagnostic connector using the Bosch KTS 500 tester

When the system is at its operating temperature and the sensor and circuit are in good order the two sets of readings should be identical. However, if the reading at the sensor is correct and the one at the ECM is not, it is reasonable to assume that there is a defect in the circuit between the sensor and the ECM. The cables and connectors should be examined for signs of damage, looseness and corrosion. With the system switched off, it should be possible to test for continuity





Fig. 7.8 Voltage test at the sensor using the Bosch PMS 100 portable oscilloscope between the ends of the signal cable and also the condition of the sensor's earth connection.

If the checks at the sensor and the ECM both produce similar defective signals, a sensor defect is indicated and an analysis of the scope patterns should give useful clues about the cause. Considerable amounts of information about the performance of the zirconia-type oxygen sensor is contained in the oscilloscope trace and the principal features are described below.

Switching characteristics

As the air-fuel ratio moves from perfect combustion (lambda = 1) to slightly rich (lambda = 0.98), the sensor voltage rises rapidly, and when the air-fuel ratio moves from lambda = 1 to lambda = 1.02, which is slightly weak, the sensor voltage drops rapidly.

This characteristic is used by the ECM to regulate the amount of fuel injected and thus to control lambda and provide the conditions for the catalyst to function, i.e. maintaining the air-fuel ratio as near as possible to the chemically correct

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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