The brief
The Polo had been creeping up the temperature gauge on motorway runs, there were fresh coolant traces near the thermostat housing on a bonnet check, and the cabin heater was slow to warm up on cooler mornings. Three pointers at the thermostat and its housing.
The thermostat is the valve that decides when coolant flows to the radiator. It stays shut while the engine warms up, then opens to let the radiator do its job. When it sticks part-closed, the engine can't shed heat fast enough on a long run, so the gauge climbs. And on this car the thermostat sits inside a plastic housing module that ages and weeps coolant from its seals, which is the wet traces. The slow heater is the same valve not regulating the way it should.
The diagnosis
A pressure test confirmed coolant was weeping externally from the thermostat housing seals. Live data on the scanner showed the thermostat not opening cleanly at its rated temperature. So it was failing two ways at once, leaking outside and not regulating inside, which made replacement the obvious call.
On this engine the thermostat isn't a part you swap on its own, it comes as one piece with its plastic housing, so the whole module gets changed with fresh seals.
The work
Enough coolant was drained to drop the thermostat housing, and the old module was unbolted and taken off. A new VAG-spec thermostat housing module went on in its place with fresh O-rings on the joints.
Then the system was refilled with the correct coolant, the air bled out the proper way so there were no pockets left, and held under pressure to confirm the seals were dry.
A road test confirmed the gauge sat steady and the heater warmed up on time.
The outcome
Gauge steady on the motorway, no coolant traces under the bonnet, and the cabin heater coming on at the right point.
The Polo went home regulating its temperature properly again. A sticking thermostat and a weeping housing only get worse, and an engine that runs hot is an engine you don't want to keep driving, so changing the module sorted the heat, the leak and the slow heater in one go.