# Heated oxygen sensor description

All E30 except M10 318i are equipped with heated oxygen sensors.

The oxygen sensor measures the oxygen content of the exhaust. The control signal it transmits is used by the DME system to regulate the proportions of fuel and air in the mixture. Together with a suitably deigned exhaust catalyst, the pollutant content of the exhaust can be reduced in this way.

In contrast to the unheated oxygen sensor, the heated sensors have a heating element controlled by a PTC (Positive Temperature Co-efficient) device. The advantage of the heated oxygen sensor is reduced time needed for the sensor to respond.

The heated oxygen sensor has a life span of 50,000 miles and uses its own relay to supply heating current. The additional heating has allowed for re-positioning of the oxygen sensor further down the exhaust, away from the manifolds. This reduces the ambient temperature at which the sensor must operate and prolongs its life. In addition, measuring accuracy is increased as the exhaust gas from the various cylinders is more thoroughly mixed by the time it reaches the sensor.

The sensor housing contains a ceramic element and two protective tubes. The front of the ceramic element is protected by a slotted tube and projects into the front tube of the catalyst, where it is surrounded by the exhaust gas flow. The rear, inner section of the ceramic element is in contact with the surrounding air. The ceramic element is made from zirconium dioxide, with a surface electrode layer of gas permeable platinum. To protect this thin layer against combustion residues, yet maintain consistent catalyst performance, a porous ceramic layer is superimposed on the platinum.

The contact spring transmits the electrical signal from the electrically conductive layer inside the ceramic element to the electrical connection.

Operating principle

The oxygen sensor acts as a measuring link in the control circuit needed to ensure efficient exhaust catalyst operation and maintain the correct fuel air mixture of 14 to 1.

Even if combustion takes place with excess fuel, a proportion of residual oxygen is present and permits information on mixture composition to be compiled.

The proportion of oxygen in the exhaust is measured and used as a control value to determine the fuel air ratio.

The sensor works according to the following principle ; At a temperature in the region of 300° C (572° F), the ceramic material begins to conduct oxygen ions. If the proportions of oxygen on the inside and outside surfaces of the ceramic element are different, an electrical voltage develops between the boundary layers and is used as a control signal.. It rises from 200 to 800 mV. On receiving this signal , the DME control unit reduces the fuel injection period, and thus the volume of fuel injected, without any significant time delay.

Warning : As a result of this operation, applying undercoat or paint to the outside of the oxygen sensor will render it inoperative.

E30 M3 Repair Manual V2.7. Copyright ©1999-2001 Koala Motorsport & Brett Anderson