Audi Workshop Service and Repair Manuals

Notes:

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.

Voltage signal for "Rich mixture" (low level of residual oxygen) is at approx. 0.6... 1.0 V.

Voltage signal for "Lean mixture (high level of residual oxygen) " is at approx. 0.0 ... +0.3 V.

A voltage jump from 0.6...1.0 V to 0.0...+0.3 V and vice versa occurs during the transition from "rich" to "lean" 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.
• ◆ 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 010).
• ◆ Lambda probe damaged by contact spray or similar product (the contact spray is drawn into the Lambda probe via the fine cavities in the electrical wiring as a result of temperature fluctuations and capillary effects).
• ◆ Lambda probe damaged by silicone vapours (engine draws in traces of any silicone based sealants that may have been used. The silicone does not burn and damages the Lambda probe).