The brief
The Q5 was running hot, specifically when it was crawling in traffic or sitting at idle, with the temperature climbing then settling once it got moving again. Overheating isn't always a faulty coolant pump, and that pattern, hot at low speed, fine on the move, points at the radiator fan. He brought it in. The radiator cooling fan pulls air through the radiator when there isn't enough airflow from the car moving, so it does its job in traffic, at idle, and on a hot day. When the fan motor fails, there's no forced airflow at low speed, so the coolant can't shed its heat and the engine overheats, until you're moving fast enough for the air to do the job instead. A failed fan motor doesn't recover, so it needs replacing.
The diagnosis
A check of the cooling system traced it to the radiator fan motor, it wasn't running when it should, the motor failed, so there was no forced airflow at low speed, which is exactly the hot-in-traffic symptom. The radiator, the coolant, the thermostat and the water pump all checked out, it was the fan motor, not the pump. That's a fan motor replacement, you don't rebuild it on the car, so the call was a complete cooling fan assembly, fitted and tested.
The work
The bits in the way came off, the failed radiator fan assembly removed, and a new genuine Audi-spec cooling fan assembly fitted, connected up, every fastener torqued to spec. With the engine warmed up the fan was checked to cut in at the right temperature and pull proper airflow through the radiator. A road test, including a spell of low-speed running, confirmed the gauge sat steady with no overheating.
The outcome
Gauge steady in traffic and at idle, the fan cutting in when it should, and no overheating at low speed. The Q5 went home with the cooling sorted. A failed radiator fan leaves the engine to overheat every time you're stuck in traffic, and an overheat can cost a head gasket, so changing the fan motor put it right before it did damage.
