GF07.04-P-6105MAR Component Description For A O2 Sensor Downstream of CAT

GF07.04-P-6105MAR Component Description For A O2 Sensor Downstream Of CAT
- refinement package
- as of model year 09

Component Identification:

Location
The O2 sensors downstream of the CAT are screwed in to the exhaust pipes downstream of the firewall catalytic converters.

Task
The O2 sensors downstream of THE CAT (guide and diagnostic sensors) detect the residual oxygen content in exhaust gas for the following tasks:
- Two-sensor control
- Monitoring of the catalytic converter effect

Design

Component Identification:

The O2 sensor downstream of CAT is designed as an insulated potential-free and planar (flat) voltage-jump oxygen sensor.
The active sensor ceramic consists of a gas permeable ceramic body made of zirconium dioxide. The inner protective tube with several slots protects the ceramic body from mechanical stresses and from temperature jumps.

Component Identification:

Function
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 signal voltage is produced at boundary surfaces as a result of particular properties of the sensor ceramic (Nernst voltage). This is the measure for the residual oxygen content in the exhaust.
The electronics in the ME-SFI [ME] control unit (N3/10) produce a so- called sensor back voltage of about ca. 450 mV at the voltage-jump oxygen sensor. For a cold voltage-jump oxygen sensor, the internal resistance of the sensor is so high that the sensor voltage is initially equal to the back voltage irrespective of the mixture composition (Lamda).

The sensor back voltage at the ME-SFI [ME] control unit can be measured against the sensor ground if the voltage-jump oxygen sensor is disconnected.

Sensor heating
In order to bring a ceramic probe body quickly up to operating temperature, the voltage-jump oxygen sensor is heated constantly with a heat output of about 7 W.
The sensor heater is actuated by the ME-SFI [ME] control unit by means of a ground signal. The heater current in the cold state is increased approximately by a factor of 4.

At coolant temperatures greater than about 20 °C and at high engine speeds, the sensor heater is switched off to prevent overheating (thermal shock).

Power is supplied to the oxygen sensor heaters through "circuit 87 M2".

Schematic diagram of the function principle

Component Identification:

Signal voltage curve

Component Identification:

The signal voltage has a steep voltage jump ( Lamda=1) at the transition from a rich to a lean mixture. This property is utilized for lambda control.