Heated Oxygen Sensor
The Heated Oxygen Sensor (HO2S) is a wide band type oxygen sensor that allows the Powertrain Control Module (PCM) to provide closed loop control of the air fuel mixture. The sensor is installed in the downpipe of the exhaust system, immediately after the connection with the turbocharger outlet.
Power for the heater element of the HO2S is provided by a battery voltage supply from the PCM relay in the Engine Junction Box (EJB). The circuit through the heater element is completed by a connection with the PCM. The PCM regulates the current flow through the heater element to control the heating of the sensor. The heater element is operated immediately after each engine start and during low load conditions when the temperature of the exhaust gases is insufficient to maintain the required sensor temperature.
The HO2S has four further connections with the PCM, two for the oxygen measuring (Nernst) cell and two for the pump cell. The PCM adjusts the current of the pump cell circuit as necessary to maintain a constant signal from the oxygen measuring cell. The PCM then relates the current of the pump cell circuit to the lambda ratio of the exhaust gases.
If the HO2S fails the PCM defaults to open loop fueling and the emissions content of the exhaust gases increases. With a failed HO2S, the engine will suffer from unstable operation and reduced performance.