I had to change the battery on my friend's 2001 Audi A4 with the 2.8L V6. When you swap out battery in the Audi, the radio will go into locked mode. To unlock it, you need an anti theft radio code which the dealer is supposed to provide it to you free of charge when you buy a new car or a 2nd hand car from an Audi dealer. My friend brought her A4 new from the dealer and as expected, the dealer never included the code in the manual. Now if you need the code, the dealer is going to ask you to pay $65 or more just to get a code from their database. Nice way to rip off your customers, Audi. The work around solution is to buy this accessory, which uses a 9VDC battery to power the settings in the radio as well as the power window settings while you replace the battery. Replacing the battery in the A4 is quite complicated as you have to remove a bunch of plastic panels to get access to the battery next to the firewall. This accessory works like a charm with the A4. The radio works, the power windows work after I swap out the OEM battery for a battery from Autozone. The price on Amazon for this accessory is cheaper than getting a similar piece from Autozone for example, however shipping can be quite expensive and this brings up the price to the same as your local Autozone, the only difference being sales tax.