In order to maintain optimum efficiency of the catalytic converters, the system endeavours to provide the ideal air-fuel mixture ratio (Lambda = 1) for combustion. 2 heated oxygen sensors (in front and behind the catalytic converter) serve as the sensors which measure the residual oxygen in the exhaust gas and transfer corresponding voltage values to the control module. Here, if necessary, the mixture composition is corrected accordingly in that the injection timing is varied. The operability of the catalytic converter (conversion efficiency) is monitored via the oxygen sensor behind the catalytic converter.
The heating resistors in the oxygen sensors are supplied with a voltage since a temperature of approx. 300 °C is necessary for effective operation of the oxygen sensors.