Heated Oxygen Sensors

HEATED OXYGEN SENSORS





Front Oxygen Sensor

The Front Heated Oxygen Sensor (HO2S) is used to supply the Engine Control Module (ECM) with information about the air/fuel mixture.

The heated oxygen sensor (HO2S) is mounted upstream of the Three-Way Catalytic Converter (TWC). The heated oxygen sensor (HO2S), which is heated electrically, produces an output voltage which is a function of oxygen content in exhaust gases.

To measure the oxygen content, the heated oxygen sensor (HO2S) requires a supply of ambient air as a reference. This reference air reaches the heated oxygen sensor (HO2S) via its cabling. This means that cabling must not be trapped or damaged in any other way. The heated oxygen sensor (HO2S) connectors must not be lubricated as the oil would interfere with the reference air.

The engine control module (ECM) will adopt substitute values if the front heated oxygen sensor (HO2S) signal is missing or faulty.

Rear Oxygen Sensor

Cars in certain markets are also equipped with a rear heated oxygen sensor (HO2S) installed downstream of the three-way catalytic converter (TWC) which measures the oxygen content of exhaust gases there.

This is to:
- Make heated oxygen sensor (HO2S) control more accurate
- Check if the front heated oxygen sensor (HO2S) has aged
- Check that the three-way catalytic converter (TWC) is working.

The rear heated oxygen sensor (HO2S) differs from the front unit in that it reacts somewhat more slowly and that the sensor element has a smaller hole.

Function

The heated oxygen sensor (HO2S) only operates above certain temperature , approx. 285°C. Normal operating temperature is in the range 350 - 850°C.

The heated oxygen sensor (HO2S) is heated electrically. One terminal is supplied with 12 V by the main relay, while a second terminal is connected to the engine control module (ECM). When this terminal is grounded, a current flows through the PTC resistor in the heated oxygen sensor (HO2S).

When the heated oxygen sensor (HO2S) is cold, the value of the PTC resistance is low and the current in the circuit is high. The engine control module (ECM) pulses the current initially to prevent damage to the PTC resistor. The resistance increases and the current falls and becomes continuous as the temperature rises. The heating period is short, approx. 30 seconds.

The heated oxygen sensor (HO2S) can be damaged if it is exposed to condensed moisture from the engine while the device is hot.

Heating of the heated oxygen sensor (HO2S) starts immediately the engine is started. When sensor temperature reaches 350°C it is maintained at a constant level until exhaust gas temperature at the front heated oxygen sensor (HO2S) and catalytic converter temperature at the rear heated oxygen sensor (HO2S) are high. This is to ensure that the temperature around the sensors is high enough to avoid water condensing onto them.

Both heated oxygen sensor (HO2S) terminals are connected to the engine control module (ECM).

When the engine is running rich, the oxygen content of the exhaust gases is low or zero. The heated oxygen sensor (HO2S) is then transmitting an output voltage of approx. 0.9 V.

When the engine is running lean, there is an excess of oxygen in the exhaust gases. The output voltage from the heated oxygen sensor (HO2S) almost drops to 0 V.

The change from high to low signal level occurs at the ideal (stoichiometric) air/fuel ratio of 14.7 kg air/1 kg fuel.

The engine control module (ECM) continuously uses the heated oxygen sensor (HO2S) signal to control fuel injection so as to maintain the ideal air/fuel ratio of 14.7 kg air/1 kg fuel.