Ecm

Part of exhaust pipe

Fig. 5.28 Diagrammatic representation of the oxygen sensor in the exhaust pipe

Part of exhaust pipe

% = 0.93 % = 1 % = 1.06 Air-fuel ratio (Approx)

Fig. 5.29 Change in sensor voltage as air-fuel ratio changes

Fig. 5.29 Change in sensor voltage as air-fuel ratio changes

Time in seconds Fig. 5.30 The voltage waveform of an EGO sensor

injected, in an ordered way, so as to keep the air-fuel ratio within the required limits. This means that the time period between peaks and valleys (frequency) of the waveform will vary with engine speed. This time period will also vary according to whether the fuelling system is single-point injection, or multi-point injection. However, the general principle holds good. The type of waveform shown can be expected from any correctly operating oxygen sensor.

The action of the oxygen sensor is dependent on its temperature. The sensor needs to reach a temperature of around 250°C before it starts to function at its best. In order to assist the sensor to reach this temperature as quickly as possible, from a cold start, it is common practice to equip the sensor with a resistive-type heating element as shown in Fig. 5.31.

This means that most oxygen sensors will be equipped with four wires: a signal wire and an earth for the sensor element, and a feed wire and an earth for the heating element. This type of sensor is known as a heated exhaust gas oxygen sensor (HEGO) and is shown in Fig. 5.32.

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