Audi A4   Controls   Instruments and warning/indicator lamps  Instruments
The gauge only works when the ignition is switched on.
When the needle reaches the reserve zone, the indicator lamp Link lights up in the instrument cluster display.
The tank capacity of your vehicle is given in the Technical data section Link.
Caution!
Never run the tank completely dry. If there is an irregular fuel supply, misfiring can occur. Unburnt fuel can then enter the exhaust system. This can lead to overheating and damage to the catalytic converter.

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