Audi A4 Owners Manual
The gauge only works when the ignition is switched on.
When the needle reaches the reserve zone, the symbol
lights up in the instrument cluster
The tank capacity of your vehicle is given in the Technical data section.
Never run the tank completely dry. The irregular supply of fuel can cause misfiring, which would allow unburnt fuel to enter the exhaust system. This can lead to overheating and damage to the catalytic converter.