O2 sensors function - GF07.04-P-4103-01M
The sensor ceramic is conductive for oxygen ions from approx. 300 °C. If the oxygen share varies greatly on both sides of the ceramic probe body, a voltage (Nernst concentration cell) is generated on the friction planes by means of the particular properties of the ceramic probe body on the friction plane. This is a measure for the residual oxygen content in the exhaust.
Curve of O 2 sensor voltage, schematic
Faults at the O2 sensor can be detected by means of the O 2 sensor signal.
Defined limit values must be maintained for the O2 sensor voltage, the duration of the period and for the sensor status change.
The O2 sensor signal has a steep voltage jump (λ=1) at the transition from a rich to a lean mixture. This property is utilized for the lambda closed-loop control.
The ground cable for the sensor signal for each O 2 sensor is routed individually up to the ME control unit.
The analysis circuit for the O2 sensor signal outputs a so-called sensor back voltage of approx. 0.470 V.
The O2 sensors are heated in order to rapidly heat up the sensor ceramic to operating temperature. The sensor heater is actuated by the ME control unit through ground. The current in the cold state is increased by approximately 4 times. The sensor heater is switched off at coolant temperatures below approx. 20 °C and at high engine speeds in order to avoid overheating (thermo shock).
If the O2 sensor is cold, the sensor internal resistance is so high that the O2 sensor voltage is initially the same as the back voltage irrespective of the mixture composition.
The back voltage at the ME control unit can be measured to the sensor ground if the O2 sensor is disconnected.