The volume of the tank on your Q5 is 23L.
When less than 1500 miles of mileage are remaining, a signal appears on your dashboard warning you to fill up. An audio signal sounds to warn you. Below 650 miles, a message warns you that restarting your car will be impossible once the tank is empty. When the tank is completely empty, a message is displayed along with three audio signals. When the tank is completely empty, pour at least 5,7L AdBlue® in the tank to turn the AdBlue® warning off. Leave the key in the ignition for twi minutes without turning the engine on.
When your tank is completely empty, you will not be able to restart the engine after it has stopped. If your engine is still running with an empty tank, you have to drive you to an AdBlue® filling station, because once you stop, the car will not start again. Fill the tank with a minimum of 5.7L, turn the ignition on without starting the engine. If the car does not start immediately, you may have to wait for two minutes with the key in the ignition for the system to detect the AdBlue® top-up.
This information can vary on some vehicles of the same model in relation to the options chosen by the manufacturer.