Audi A3 Coolant Problems

Hi All,

I have a 54 reg audi a3 1.9 TDI and I have a red coolant warning on my dashboard constantly when I drive.

My car isnt loosing coolant and the temp gauge is ocntantly on 90 degrees but this computer warning just wont go off!!!

I took it to my Audi dealership today for tests and they did pressure tests and filled up the coolant as they said it was a little low and said that all the warning lights were off and there were no leaks at all within the engine.

I drove the car off the forecourt and 1 minute later the warning light for the coolant came on again and wont go off.

Is there anyone else who has had the same problems, is there anyting that you could think it mat be as it will be getting took back to Audi tmr but I want to know what I should be looking for so I dont get over charged, I alreday paid them £30 today for some coolant and tests so hopefully, all for nothing!!!!