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Hi guys doing this for a friend, his A6 is losing water at the expansion tank Audi suggested thermostat, changed it but no difference, any 1 any idea what it could be

Thermostat should not create coolant loss. You would be having significant mechanical/overheating problems if this were the case, its an in-line component so thats a complete waste of money.

I'd check the usual suspects, gaskets - how's the oil filler cap and dip stick looking? any signs of loss from around the expansion vessel? what about connection pipes?? Visual of the rad, any damage? How much coolant is being lost? Any signs on the drive/garage floor?

Yes it is on the garage floor, although not a great deal. Seems to be mainly around the overflow of the header tank. Audi Techs checked dip stick and oil cap, and said everything was ok. It does not seem to be coming from the pipes or the rad. Also no problems with overheating, the temp gadge sitting at 90, however, water pipes seem like they have alot of pressure in them. Any other suggestions, thankyou Bayliner.

Get a block test done , This will indicate if there is a head gasket failure .