The brief
The CLA180 had frequent low-coolant warnings, the engine running hotter than it should, puddles of coolant under the car, a sweet smell inside and out, and the odd wisp of steam from the bay. He brought it in before it overheated outright. That whole list says the cooling system is losing coolant 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, which is the puddles and the sweet smell. Lose enough and the level drops and the engine can't keep its temperature, which is the overheating and the steam. A cracked plastic housing only cracks further, and an engine that overheats can warp a head, so it needed tracing and fixing rather than topping up.
The diagnosis
A pressure test confirmed coolant weeping from the thermostat housing, a hairline crack in the plastic, with the hoses, the water pump, the radiator and the expansion tank all checking out clean. Since the housing and thermostat on this engine come as one piece, the 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 gauge sat steady, and there was nothing weeping.
The outcome
No more low-coolant warnings, no puddles, no sweet smell, the level holding, and the gauge steady. The CLA180 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 kept it to a planned job.