The brief
The E200 had a coolant warning light on the dash, the owner had spotted brightly-coloured coolant puddles under it after parking, and he was finding himself topping up the reservoir more often than he should. He brought it in before it overheated. Coolant doesn't go anywhere on its own, so a level that keeps dropping and puddles under the car means it's leaking somewhere. On these Mercedes engines the thermostat sits in a plastic housing up on the front of the engine, and that plastic ages and cracks, so coolant weeps out from there, a little every time the engine runs. Catch it early and it's a tidy job; leave it and the slow loss becomes a fast one and an engine that runs out of coolant overheats, which on these can cost a head gasket.
The diagnosis
A pressure test confirmed coolant weeping from the thermostat housing, a hairline crack in the plastic, with the hoses, the radiator and the expansion tank all checking out clean. The thermostat in it had also lost a bit of travel, so the warm-up wasn't quite right either. Since the housing and thermostat on this engine come as one piece, the sensible fix is to change the assembly complete with a fresh seal rather than try to patch a cracked plastic part.
The work
Enough coolant was drained to drop the thermostat housing, the old cracked unit removed, and a new genuine Mercedes-spec housing fitted with a fresh seal. The system was refilled with the correct coolant, the air bled out the proper way so no pockets were left, and held under pressure to confirm the seals were dry. A road test confirmed the level held, the warm-up was back to normal, and there was nothing weeping.
The outcome
No more coolant warning, no puddles after parking, the level holding, and the gauge steady. The E200 went home with the cooling system sealed up. A cracked plastic housing only cracks further, and the failure at the end is an overheat that can cost a head gasket, so tracing the leak and changing the unit before it let go kept it to a planned job.