The brief
The owner kept seeing the temperature needle creep up in slow Singapore traffic on the A6, and the aircon would lose its bite once the engine got hot. On the open road, both were fine.
That split is the tell. At speed there is plenty of air flowing through the front of the car to cool the radiator and the aircon condenser on its own. In stop-go traffic that air stops, and the radiator fan is supposed to take over and pull air through. Fine on the move, hot at a standstill, is what you get when the fan is not running. A dead fan, in other words.
The diagnosis
We let the engine get hot and watched the fan. It did not spin. The workshop bench-tested the fan motor: dead, no response.
The wiring and the fan control module checked out clean, which ruled out a circuit fault or a control problem and left the fan assembly itself as the failed part. There is nothing to repair on a seized fan motor; the fix is replacement.
The work
Removed the front bumper enough to get at the fan shroud, which sits behind the radiator, then dropped the failed fan assembly out. Fitted a new Audi-spec radiator fan, reconnected the harness, and refitted the bumper.
Then ran a heat test: let the engine warm up to the threshold and confirmed the fan kicked in at the correct temperature, and that it stepped up to high speed when the AC compressor came on. A fan job you do not heat-test is a fan job you have not finished.
The outcome
Temperature held steady through a full traffic test. The aircon stayed cold the whole way. The fan stepped up to high speed when the AC compressor engaged.
The A6 went home with cooling performance back to standard. For the owner, in this climate, that is not a luxury: a car that runs hot and loses its aircon in a traffic jam is exactly the one you do not want on a busy Singapore evening.
And catching a seized fan before it cooked the engine in a long jam kept this to a fan replacement, rather than the much bigger bill an overheating event brings.