The volume of the tank on your A4 Avant is 12L.
In case your AdBlue® tanks reaches a remaining mileage of 1500 miles, a warning light appears on the driver's screen An audio signal sounds to warn you. Below 650 miles of remaining mileage, an AdBlue® warning is visible on the dashboard, a alarm signal can be heard and a message warns the drivers that restarting the car is impossible once the tank is empty. When the tank is empty, a message is displayed along with three audio signals. To stop the AdBlue® warning when the tank is empty, pour at least 5,7L of AdBlue® in the tank.
If your warning shows a remaining mileage of 0 miles, restarting your car is impossible once the engine has been turned off. If your engine is still running and your tank is empty, it is still possible to drive to a gas station. If the car does not start after the AdBlue® tops up, wait for two minutes for the system to detect it. If the AdBlue® tank is empty, turn the ignition on for about 15 seconds and then start the engine.
This information can vary on some vehicles of the same model in relation to the options chosen by the manufacturer.