O2 sensors function - GF07.04-P-4103-01AM
The sensor ceramic is conductive for oxygen ions from approx. 300°C. If the oxygen concentration on both sides of the sensor ceramic differs, a voltage is produced at the boundary surfaces as a result of the particular properties of the sensor ceramic (Nernst voltage). This is a measure for the residual oxygen content in the exhaust.
The O2 sensor signal has a steep voltage jump (λ=1) at the transition from a rich to alean mixture. This property is utilized for the lambda closed-loop control.
Curve of O 2 sensor voltage (schematic)
Faults at the O2 sensor can be detected by means of the O2 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 ground line for the sensor signal is routed individually for each O2 sensor up to the ME-SFI [ME] control unit (N3/10).
The analysis circuit for the O2 sensor signal (in the ME-SFI [ME] control unit) outputs a so-called back voltage of approximately 0.450 V. In case of a cold O2 sensor, the internal resistance of the sensor is so high that irrespective of the mixture composition, 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.
The O2 sensors are heated to warm up the sensor probe bodies to operating temperature quickly. The sensor heaters are actuated by the ME-SFI [ME] control unit via ground. The current in the cold state is increased approximately by the factor 4.
At coolant temperatures greater than about 20°C and at high engine speeds, the sensor heaters are switched off to prevent overheating (thermal shock) of the O2 sensors.