Oxygen Sensor: Description and Operation

HEATED OXYGEN SENSORS

Front Heated Oxygen Sensor
The front HO2S is used to supply the ECM with information regarding the composition of the air/fuel mixture.

The Heated Oxygen Sensor (HO2S) is positioned in the exhaust pipe ahead of the Three-Way Converter (TWC). The HO2S probe, which is electrically heated, produces an output voltage whose level varies with the oxygen content in the exhaust gases.

To measure the oxygen content, the HO2S requires a supply of ambient air as a reference. Since this is supplied through the wiring, the lead must not be clamped or damaged in any other way.

Neither is it permitted to grease the HO2S connectors, since the oil in the grease would interfere with the reference air

Rear Heated Oxygen Sensor
Cars in certain markets are also equipped with a rear HO2S, which is installed downstream of the TWC and measures the oxygen content of the gases at that point.

The purposes are to:
^ make the fuel trim function more accurate;
^ monitor the front HO2S for ageing;
^ monitor the operation of the TWC.

The rear HO2S differs from the front unit in that it reacts somewhat more slowly and that the sensor element is slightly different in appearance.

Principle of Operation
The HO2S operates only above a certain temperature (approx. 285°C / 545°F). The normal working temperature ranges from 350 to 850°C (662 to 1562°F).

The sensor is heated electrically. One terminal is supplied at 12 Volts by the main relay, while a second terminal is connected to the ECM. When this terminal is grounded, a current flows through the PTC resistance in the sensor. When the HO2S is cold, the value of the resistance is low and the current in the circuit is high. (To prevent damage to the resistance, the ECM pulses the current initially.) As the temperature in the PTC resistor rises, so too does resistance increase in the resistor, the current drops and gradually switches to non-pulsed current. The heating period is short (approx. 30 seconds).

The HO2S may be damaged if it is exposed to condensed moisture from the engine while the device is hot.

B5234T (model year 94): The control module waits a while before HO2S heating is started; for the front HO2S, up to 1 minute, and for the rear HO2S up to 7 minutes. This allows the temperature around the HO2S units to rise, thus preventing the risk of condensate.

B5234T/B5234T5 (model year 95): HO2S pre-heating starts immediately after engine start, but can be interrupted during the first three minutes if the engine slows to idling speed.

Heating of the rear HO2S is delayed by up to 7 minutes. This is done so as to ensure that the temperature around the HO2S units has had time to rise, thus avoiding any risk of condensation.

B5254S: HO2S pre-heating begins immediately after engine start. When HO2S temperature reaches 350°C (662°F), HO2S temperature remains constant until exhaust gas temperature at the front HO2S and TWC temperature at the rear HO2S reach levels where there is no risk of condensation.

Both sensor terminals are connected to the ECM

When the engine is running rich (lambda<1), the oxygen content of the exhaust gases is low or zero and the output signal is approx. 0.9 Volts.

When the engine is running lean (lambda>1), the exhaust gases contain excess oxygen and the output signal falls to almost 0 Volt.

The change from high to low signal level occurs at the ideal (stoichiometric) air/fuel ratio of 14.2 kg/lkg (lbs / 1lb).

The ECM uses the HO2S signal to control the fuel injection so as to maintain the ideal value of lambda=1.