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 1.0 V.

The voltage signal for "Mixture too lean" (high residual oxygen) is between about 0.0 and +0.3 V.

The transition from "rich" to "lean" is accompanied by a voltage jump from between 0.7 and 1.0 V to between 0.0 and +0.3 V, 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 wire or earth wire.
• ◆ Lambda probe too cold; lambda probe heating not working.
• ◆ Lambda control switched off (control unit has registered a fault in the injection system; interrogate fault memory and read measured value block, Display Group 99).
• ◆ 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.)