Audi Workshop Service and Repair Manuals

The lambda probe compares the oxygen content in the air with the residual oxygen content in the exhaust gas and sends a voltage signal to the control unit.

The voltage signal for "Mixture too rich (low residual oxygen)" is between about 0.7 and 1.1 V (based on reference earth).

The voltage signal for "Mixture too lean (high residual oxygen)" is between about -0.1 and +0.3 V (based on reference earth).

The transition from "rich" to "lean" is accompanied by a voltage jump from between 0.7 and 1.1 V to between -0.1 and +0.3 V (based on reference earth) and vice versa (λ = 1.0).

Because of the sharp voltage jump the lambda control cannot maintain a constant ideal mixture composition of λ = 1.0. The system fluctuates continuously between "slightly too lean" and "slightly too rich".

If the voltage does not change or only changes slowly the following faults are possible:

• ◆ Slots or holes in probe head are blocked.
• ◆ Lambda probe has been subjected to excessive thermal stress.
• ◆ Contact resistance in signal wiring or reference earth wiring.
• ◆ Lambda probe too cold; lambda probe heating not working.
• ◆ Lambda control switched off (control unit has detected a fault in the injection system); =>Interrogating fault memory and Reading measured value block, Display Group 21.
• ◆ Lambda probe damaged by contact spray or similar product. (The contact spray is drawn through the fine cavities in the electrical wiring as a result of thermal fluctuations and capillary effect.)
• ◆ Lambda probe damaged by silicone vapours. (The engine draws in traces of any silicone-based sealants that may have been used. The silicone does not burn and damages the lambda probe.)