Oxygen Sensor: Description and Operation

Front Heated Oxygen Sensor, (592), ME9

Location

Main task
To monitor exhaust gas composition with regard to oxygen content.







Type
Broadband lambda sond.

Connection







Diagram







Function
The broadband oxygen sensor is significantly more complex than a narrow band oxygen sensor which is normally used for petrol engines.

In principle the sensor consists a heated narrow band sensor, a so-called Nernst cell, which is connected to a pump cell. The cells are produced from ZrO2 with porous platinum electrodes. The cells are positioned at a distance of 10-50 micrometres from each other, this gap is connected to the surrounding exhaust gases via a small hole.

The pump cell pumps oxygen from the measured gap to the surroundings if the exhaust gases have a lambda value above lambda=1. When the value lambda=1 is reached the voltage from the Nernst cell is 450 mV, a voltage that the ECM attempts to hold constant by varying the pump current. Consequently, the size of the current through the pump cell is a measure of the oxygen composition of the exhaust gases. A high lambda value in the exhaust gases results in a relatively high positive pump current.

The temperature of the oxygen sensor must be regulated precisely. ECM does so by varying the preheating current in the heating element.

Amongst other things, the sensor is used to ensure the function of the mass air flow sensor and the injectors.