O2 Sensors, Function

O2 Sensors, Function




1 Sensor ceramic
2 Electrodes at boundary surfaces
3 Exhaust pipe
Arrows
black: exhaust gases
white: outside air

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 O2 sensor voltage, schematic
A minimum sensor voltage (lean mixture)
B maximum sensor voltage (rich mixture)
C Lower limit value for work measurement
D Upper limit value for work measurement
E Duration of sensor state change for lean-rich transition
F Duration of sensor state change for rich-lean transition
G Duration between sensor state change
H Threshold for rich range (e.g. greater than 475 mV)
I Threshold for lean range (e.g. less than 425 mV)
T Time in ms
U Sensor voltage in mV

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 O2 sensor signal has a steep voltage jump (at lambda=1) at the transition from a rich (A) to a lean mixture (B). This property is utilized for the lambda closed-loop control.





ME-SFI 1.0
The ground for the sensor signal at the ME 1.0 control unit is raised to approx. +0.71 V compared to the general vehicle ground. In the event of a short circuit of the sensor signal to ground, the ME control unit thus switches the lambda closed-loop control to open-loop mode (average value). This prevents a high fuel consumption.

ME 2.0, ME 2.1, ME 2.7, ME 2.8 with engine control unit for EURO 3/4
The ground cable for the sensor signal for each O2 sensor is routed up to the ME control unit.
The ground for the sensor signal is increased to approx. +0.268 V compared to the general vehicle ground e.g. at the ME 2.1 control unit. In the event of a short circuit of the sensor signal to ground, the ME control unit thus switches the lambda control to open-loop mode (average lambda value). This prevents a high fuel consumption.
The analysis circuit for the O2 sensor signal outputs a so-called sensor back voltage of approx. 0.470 V. If the O2 sensor is cold, the sensor internal resistance is so high that the O2 sensor voltage is initially equal to the backvoltage irrespective of the mixture composition.
The backvoltage at the ME control unit can be measured to the sensor ground if the O2 sensor is disconnected.