Functional description

The front oxygen sensor measures residual oxygen in the exhaust gas. The fluctuating values of the residual oxygen are forwarded to the engine control unit as a current signal. The DME corrects the mixture composition by means of fuel injection. A second oxygen sensor (monitoring sensor) is built in behind the catalytic converter. The catalytic converter has a high oxygen storage capacity. This means there is only a little oxygen behind the catalytic converter. The monitoring sensor supplies a virtually constant (attenuated) voltage. With increasing age, the oxygen storage capacity of the catalytic converter declines. The monitoring sensor then reacts increasingly to oxygen sensor deviation with voltage fluctuations. These characteristics are used by a special diagnostic function for catalytic converter monitoring. A malfunction of the catalytic converter is indicated by the emissions warning light.

Fig 1: Oxygen Sensor, Housing And Plug Connection

Courtesy of BMW OF NORTH AMERICA, INC.

Example schematic diagram

As residual oxygen remains in the exhaust gas during operation with rich mixture, an electrical voltage is present between the outer electrode and the inner electrode. To ensure that the reference size always remains the same, the reference air duct is connected to the ambient air. The reference size is therefore the residual oxygen content of the ambient air.