over heating signal

My lovely A6 is showing the Temp warning sign on the dash ! Its a 1997 model and i have recently replaced the water pump unit when it started over heating. Then a few days later warning light came on again but the temp gauge was not going over 90 so i replaced the engine heat sensor ! no faults are showing up on the digisnostics computer ??? Im lost now when engine is hot i can open the coolant tank and not muc presure is released which might telll me that the head is not gone no smpke from exhaust etc

Any ideas will be greatfully welcomed i can only think of electrics as a problem or the fact that the water pump went might tell me water was used instead of coolant which might hav caused some hot spots around some parts of engine

Many thanks