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 level of residual oxygen) is between about 0.7 and 1.0 V (referenced to reference ground).

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

During the transition from "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".

When the engine is at normal operating temperature the control frequency when the engine is idling is at least 0.5 Hz (corresponds to 30 oscillations per minute). At fast idling (2500...2800 rpm) the control frequency should be at least 1 Hz (corresponds to 60 oscillations per minute). An oscillation comprises the voltage jump from the maximum value through the lowest value back to the maximum value.

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.
• ◆ Probe poisoned by leaded fuel, test with "PLUMBTESMO" test strip for lead content.
• ◆ Contact resistance in signal wiring or reference ground line.
• ◆ Lambda probe too cold; lambda probe heating not working.
• ◆ Lambda control switched off (control unit has detected fault(s) on the fuel injection system =>Interrogate fault memory and Read measured value block, Display group 21).
• ◆ 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 traces do not burn and damage the lambda probe).