Component description for an oxygen sensor - GF07.04-P-6101CZ

ENGINE 272.911 in MODEL 204.0 up to Model Year 2008 Lambda probe after catalytic converter

ENGINE 272.921 in MODEL 204.0 /2 up to Model Year 2008 Lambda probe after catalytic converter

ENGINE 272.947 in MODEL 204.0 /2 up to Model Year 2008 Lambda probe after catalytic converter

ENGINE 272.948 in MODEL 204.0 up to Model Year 2008 Lambda probe after catalytic converter

ENGINE 272.961 in MODEL 204.0 /2 up to Model Year 2008 Lambda probe after catalytic converter

ENGINE 272.971 in MODEL 204.0 up to Model Year 2008 Lambda probe after catalytic converter

Location

Fig 1: Locating Oxygen Sensors

Courtesy of MERCEDES-BENZ USA

Oxygen sensors downstream of catalytic converter (KAT) are screwed in to the side in the firewall catalytic converters. They project into the hollow spaces between both catalytic converter inserts of the firewall catalytic converter.

Task

Oxygen sensors downstream of TWC (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

Fig 2: Oxygen Sensor Design

Courtesy of MERCEDES-BENZ USA

Oxygen sensors downstream of TWC are insulated as voltage free and designed a planar (flat) wideband oxygen sensors.

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.

Connector

Fig 3: Identifying Oxygen Sensor Connector Terminals

Courtesy of MERCEDES-BENZ USA

Function (schematic)

Fig 4: Oxygen Sensor Function Diagram

Courtesy of MERCEDES-BENZ USA

Signal voltage

Fig 5: Oxygen Sensor Signal Voltage Graph

Courtesy of MERCEDES-BENZ USA

The signal voltage has a steep voltage jump at the transition from a lean to a rich mixture (λ=1)

This property is utilized for lambda control.

The sensor ceramic is conductive for oxygen ions from approx. 300°C. If the oxygen concentration on both sides of the sensor ceramic differs, the signal voltage is produced at the boundary surfaces as a result of the particular properties of the sensor ceramic (Nernst concentration cell). This is a 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 the 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 (λ).

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

Sensor heating

In order to bring ceramic probe bodies quickly up to operating temperature the voltage-jump oxygen sensors are heated constantly with a heat output of about 7 W.

The sensor heaters are 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, sensor heaters are switched off to prevent overheating (thermal shock).