The computer is showing a turbo drive fault.
Hi A2A2,
Sorry to hear about the problems you're having with your A2.
There are absolutely no electronic sensors attached to the A2's turbo. If AVIT have told you that "the computer is showing a turbo drive fault", I suspect they're either out of their depth, feeding you nonsense, grasping at straws, dumbing things down, catastrophising or some similar explanation. I also cannot see how the exhaust manifold would "come adrift". I wouldn't wish to be too critical of a company with whom I've never had any dealings, but the second-hand information we're getting from them is not inspiring confidence. Quite the reverse!
There are loads of reasons why the car might be going into limp mode. There could be a problem wih the variable nozzle control unit or any of the associated pneumatic/vacuum tubing. There could be a split in a rubber pipe carrying the turbo's compressed air. The intercooler could have a split in it, as can be seen in JustJust73's post above. The MAP sensor could be faulty. I could continue to list reasons, but we'd be here a while. Although the TDI90's turbo is known for its fragility, replacing the turbo itself should be the absolute last resort, only once every other possible cause for the car's fault has been eliminated.
Have AVIT shown you the diagnostic scan results? When the car goes into limp mode, a fault code is stored in the engine management unit. The fault code can be vague, but it'd certainly help to narrow things down. On a vast majority of occasions, limp mode is trying to protect the turbo from failure rather than being caused by turbo failure.
If a new turbo is needed, don't get one from the Audi dealerships. I kid you not, they add a margin of £600 or more. The TDI90's turbo was made by KKK, which has since been bought by BorgWarner (a huge player in the turbo manufacturing market, alongside Garrett, etc). Buying directly or from one of their retailers will save you a small fortune.
May I ask, where is the car now?
Cheers,
Tom