Heated Oxygen Sensors (Ho2S)

Front heated oxygen sensor (HO2S) 

Courtesy of VOLVO CARS CORPORATION

Front heated oxygen sensor (HO2S) is used to provide the engine control module (ECM) with information about the remaining oxygen content of the exhaust gases before the three-way catalytic converter (TWC). This is so that the engine control module (ECM) can continually check the combustion so that λ=1 is achieved. λ=1 is the ideal fuel-air ratio, with 14.7 kg air per 1 kg fuel.

The heated oxygen sensor uses current regulation and its signal characteristic is linear. With a linear signal characteristic, the amplitude of the signal curve is low when changing the oxygen content in the exhaust gases. The probe consists of a preheating element (see "Preheating heated oxygen sensors (HO2S) " below) and the actual lambda sensor. The lambda sensor is an oxygen sensitive ceramic body consisting of zirconium oxide. The control module supplies power to the ceramic body, which reacts to the oxygen content of the exhaust gases. This in turn affects the signal to the engine control module (ECM). In order to determine the oxygen content in the exhaust pipe, the heated oxygen sensor needs reference air from the surrounding air. This reference air reaches the heated oxygen sensor via the air lines.

The heated oxygen sensor can be diagnosed by the engine control module (ECM), and signals can be read out. For further information, see: HEATED OXYGEN SENSOR (HO2S) DIAGNOSTIC

The control module can be used to read off the calculated lambda value from the heated oxygen sensor (HO2S) signal.

Rear heated oxygen sensor (HO2S) 

Courtesy of VOLVO CARS CORPORATION

The rear heated oxygen sensor is used to give Engine control module (ECM) information about remaining oxygen content in the exhausts behind the catalytic converter. The information is used by Engine control module (ECM) to check the catalytic converter's function. This check is performed when conditions for catalytic converter diagnosis are fulfilled.

Rear heated oxygen sensor has no direct impact on control of the fuel/air mixture, however, Engine control module (ECM) uses the signal to optimize the signal from the front heated oxygen sensor.

Before the catalytic converter and the rear heated oxygen sensor have become warm, the center heated oxygen sensor is used instead of the rear heated oxygen sensor.

For further information, see: THREE-WAY CATALYTIC CONVERTER (TWC) DIAGNOSTICS

The heated oxygen sensor (HO2S) uses voltage control. The signal characteristic is binary. With a binary signal characteristic, the amplitude of the signal curve changes considerably when changing the oxygen content in the exhaust gases. Otherwise its components and function are the same as the front heated oxygen sensor (HO2S).

The heated oxygen sensor can be diagnosed by the engine control module (ECM), and signals can be read out.

Centre heated oxygen sensor 

Courtesy of VOLVO CARS CORPORATION

The center heated oxygen sensor is of the same type as the rear heated oxygen sensor, but with higher rated heating coil. This contributes to faster warming of the sensor, which is necessary since this function is to work immediately after engine start when the catalytic converter and the rear heated oxygen sensor are not yet warm.

When the catalytic converter and the rear heated oxygen sensor have become warm, the rear heated oxygen sensor is used instead of the center heated oxygen sensor.

The center heated oxygen sensor has no direct impact on control of the fuel/air mixture, however, Engine control module (ECM) uses the signal to optimize the signal from the front heated oxygen sensor.

For further information, see: THREE-WAY CATALYTIC CONVERTER (TWC) DIAGNOSTICS

The heated oxygen sensor (HO2S) uses voltage control. The signal characteristic is binary. With a binary signal characteristic, the amplitude of the signal curve changes considerably when changing the oxygen content in the exhaust gases. Otherwise its components and function are the same as the front heated oxygen sensor (HO2S).

The heated oxygen sensor can be diagnosed by the engine control module (ECM), and signals can be read out.

Preheating of the heated oxygen sensors (HO2S) 

The heated oxygen sensor (HO2S) only functions above a certain temperature, approximately 300 °C. The normal operating temperature is between 300-900 °C. The heated oxygen sensors (HO2S) are electrically pre-heated so that operating temperature is rapidly reached. They are also pre-heated to ensure that the heated oxygen sensors (HO2S) maintain a normal operating temperature and to prevent condensation which could damage the heated oxygen sensor (HO2S).

The sensor's heating coil consists of a PTC-resistor. The heating coil is supplied with voltage from the system relay and is grounded internally in the Engine control module (ECM).

When the control module grounds the connection, a current will pass through the PTC-resistor. When the heated oxygen sensor is cold, the resistance in the PTC-resistor is low and a high current will pass through the circuit. To avoid condensation damage to the heated oxygen sensor the current is pulsed from the Engine control module (ECM) in the beginning. Depending on the temperature, consideration is given to dew point and, as the temperature increases in the PTC-resistor, the resistance increases in the resistor, the current is reduced and transfers gradually to non-pulsing current.

The heating period for the front heated oxygen sensor is short, approx. 20 seconds.

The heating coil in the center heated oxygen sensor is slightly pore powerful than that in the rear, to obtain extra fast warming.

The heater element heats the heated oxygen sensors (HO2S) to approximately 350 °C. The probes maintain this as a minimum temperature.

The engine control module (ECM) can diagnose the heater element.