I tried the same thing of switching the heater on in the afternoon and nothing happened. I don't know if it was just a coincidence.
My audi A4 1.8T (2005) overheats in the mornings when I switch on the heater? on 2006 Audi A4
Must be the gauge circuit or another problem, heater is not actually the trouble. Have coolant level checked.
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poss water pump impellar rotting away,very common