So if you're reading this, you probably don't know how much fuel you have left except for guessing by your odo, can't tell how hot your engine is and feel like you have 140/20 vision when you try and read your center display. Yep, your cluster is SCREWED.
A this point you have basically three options:
1. Dealership. Unless you can somehow finagle your way into getting a freebie repair from a dealer, and yes it has been done, you will be paying $1000+. Sorry. But you'll have a working cluster again.
2. Buy a replacement. This is where it gets sticky.
First, you have to make sure your cluster will be compatible with your car in general. I believe somewhere in 2002 Audi added CAN-bus capability which cut off compatibility of older clusters with newer cars. I'm not entirely sure. But you'll wanna check that out, usually speaking to a knowledgeable Audi tech can help.
Second, you have to worry about immobilizer. Somewhere in 2001, the TT got fitted with VAG's Immobilizer III technology. Immobilizer III requires that the ECU, cluster and key have an identical code programmed into them for the car to start. Up until '01, TT's came with Immobilizer II which only requires the key and ECU to match. Obviously, switching in a new cluster will give you a different code. Usually, you can determine if you have Immo III by turning the key to accessory power and seeing if you get a little key icon on the gauge face. Sure fire way to determine whether you have it or not is to remove the cluster itself and see if the car starts.
So your options for dealing with immobilizer consist of getting your cluster reprogrammed or getting an immobilizer defeat. The latter is $150 from APR.
Not sure if you can deal with Immo III through VAG-COM, check out Ross Tech's info about it:
Third, you have to get your mileage reprogrammed if you want to have an accurate odometer. The cluster will read the mileage IT came with, not your car. You CAN NOT do this with VAG-COM unless the cluster has <62 miles on it. Dealers I've talked to have been very vague on what they can do, but they pretty much told me they had no guarantee that they'd be able to change it.
For some reason, even high end cluster tools like VAGdashCOM (not VAG-COM, VAGdashCOM is an $800 tool that has to be imported from Germany) can not program TT clusters. A friend of mine who has programmed countless MkIV clusters could not even read TWO different used TT clusters I brought to him or the original one from my car.
At this point I think there is no way to fix mileage on a TT cluster short of having a dealer try (with whatever they use). Since my car is Immo II, I just swapped in a used cluster and left the mileage alone while taking note of my old mileage. My 90,000 mile car's odo reads around 25,000...I can deal with that.
So if you can get all that taken care of, you will have a successful cluster swap.
3. BBA-Remanufacturing. These guys rebuild your original cluster instead of replacing it. I haven't heard much about them, and the little I have heard hasn't been good. You could give them a shot I suppose; they claim to fix all gauges and attempt to fix the center display but it's not guaranteed.
So there are your main options. Pick your poison.