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 rich" (low residual oxygen) is between about 0.7 and 1.1 V (referred to reference earth).

The voltage signal for "Mixture weak" (high residual oxygen) is between about - 0.1 and +0.3 V (referred to reference earth).

During the transition from 2 "rich" to "lean" and vice versa

(λ = 1.0), there is a voltage jump from 0.7 ... 1.1 V to -0.1 ... +0.3 V or vice versa.

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 thermally overloaded.
• ◆ Probe poisoned by leaded fuel, test with "PLUMBTESMO" test strip for lead content.
• ◆ Contact resistance in signal wiring or reference earth line.
• ◆ Lambda probe too cold; lambda probe heating not working.
• ◆ Lambda control switched off (control unit has recognized a fault in the injection system => interrogate fault memory).
• ◆ Lambda probe damaged by contact spray or similar product. (The contact spray is drawn into the probe through the fine cavities in the electrical wiring as a result of thermal fluctuations and capillary effects).
• ◆ Lambda probe damaged by silicone vapours. (The engine draws in traces of any silicone based sealants that may have been used. These silicone constituents are not burnt and damage the lambda probe).