The brief
The GLA180 came in with a thermostat that was misbehaving on two fronts. The owner had noticed coolant traces around the thermostat housing, so something was weeping there, and the engine was being slow to warm up, with the cabin heater taking longer than usual to give any real heat and a check-engine light on.
The thermostat is the valve that decides when coolant gets to flow through the radiator. Cold, it stays shut so the engine warms quickly; up to temperature, it opens so the engine doesn't overheat. When it sticks open, coolant goes to the radiator too soon, the engine never quite gets to its proper running temperature, and you feel it as weak heat from the vents and the engine working less efficiently than it should.
The diagnosis
A pressure test confirmed the external part: coolant was weeping from the seal at the thermostat housing.
On the scanner there was a stored fault code, P0128, which in plain terms means the coolant temperature was sitting below where the thermostat is supposed to hold it. That's the signature of a thermostat stuck open, letting too much heat away too early. So we had two faults in one part, a leak at the housing and a thermostat not regulating.
With the housing weeping and the thermostat itself failed, the right fix is the complete housing module, not a half-measure.
The work
We drained enough coolant to drop the thermostat housing, took off the old module and fitted a new Mercedes-spec thermostat housing complete with a fresh seal.
The system was refilled with the correct coolant and bled properly to clear any trapped air, then held under pressure to confirm the new housing and all its connections were sealed.
A run afterwards confirmed the engine came up to temperature at the right point, the heater warmed up properly, and there were no coolant traces around the housing.
The outcome
The engine reaches and holds its proper running temperature now, the cabin heater warms up when it should, and there are no coolant traces around the housing. The stored code was cleared and stayed clear.
The GLA180 went home regulating its temperature properly again, running at the efficiency it's meant to and with the leak sealed at the source.