The brief
Joe brought his A3 in for a service and mentioned the coolant level low warning had been prompting. On the inspection the team found the coolant pump module, which integrates the thermostat, had corroded from the inside. He had it sorted while the car was in. The coolant pump circulates coolant through the engine and the radiator so the heat goes somewhere, and on this engine the pump and the thermostat come as one module. The internals can corrode over time, the impeller and the housing, so the pump moves less water and the seal weeps coolant, hence the dropping level. A corroded pump module doesn't recover, and you change the whole unit so the thermostat goes with it. So it needed replacing.
The diagnosis
A pressure test on the cooling system pinpointed it, the coolant pump module was weeping and corroded from within, the impeller and housing degraded, which is the disappearing coolant. The radiator, the hoses, the expansion tank and the rest of the system held fine. The routine service items were due alongside. That's a pump module replacement, with the integrated thermostat and a fresh seal, done with the service.
The work
The engine oil and filter were changed and the other service items done. The cooling system was drained, the old corroded coolant pump and thermostat module removed, and a new genuine Audi-spec module fitted with a fresh seal and the drive belt set back up properly. The system was refilled with the correct Audi coolant, the air bled out the proper way, and held under pressure to confirm the seals were dry. A road test confirmed the gauge sat steady through traffic and at speed with no overheating, the coolant level held, and nothing leaking.
The outcome
No more coolant loss, no low-coolant warning, the level holding between checks, the gauge steady, and the service done alongside it. The A3 went home with the cooling system circulating properly again. A corroded coolant pump module weeps and moves less water, and the failure at the end is an overheat, so catching it during the service and changing the module kept it to a tidy job.
