Heated Oxygen Sensor (HO2S)

Sensor monitors the amount of oxygen in exhaust gas. The heater portion of this sensor quickly brings sensor to operating temperature. On G20, one front (before 3-way catalyst) and one rear (after 3-way catalyst) sensor is used. On Q45, 2 front and 2 rear sensors are used.

On all models, front HO2S provides a voltage signal which is used by ECM to obtain optimum air/fuel mixture. HO2S returns a sweeping return signal which varies between.25 and.75 volt. A rich exhaust gas mixture causes high sensor voltage (0.6-1.0 volt). A lean exhaust gas mixture causes low sensor voltage (0.1-0.3 volt). The ECM monitors signal from sensor and adjusts the injector pulse width accordingly.

Rear HO2S is used to monitor catalyst efficiency. If switching characteristics of front HO2S are shifted, air/fuel ratio is controlled by rear HO2S input. Under normal conditions, rear HO2S is not used for engine control.