Coolant leaks on the 1.5 TGDI thermostat housing

The thermostat housing on 1.5 TGDI engines is glass-fibre reinforced plastic, and past roughly 60,000 km it loses flexibility around the temperature sensor mount — that's where a micro-crack usually starts.

The early symptom is a faint coolant smell in the engine bay and a white residue on the housing, usually only visible on close inspection with a flashlight. Left alone, the crack grows into a visible leak and a dropping coolant level.

Replacing the housing with its gasket takes about 40 minutes and doesn't require draining the whole system — lowering the level below the housing's port is enough.