The brief
The A200 had been overheating, the temperature gauge climbing, with coolant leaks, the owner topping up often, steam from the engine bay, and a grinding or whining noise from the engine. He stopped driving it and brought it in, which is the right call, an engine that overheats shouldn't be run on. That whole list points at the coolant pump. The pump circulates coolant around the engine and through the radiator to carry the heat away, and when it fails it can leak coolant from its seal, which is the leak and the steam, it stops moving coolant properly, which is the rising temperature, and its bearings wear, which is the whine. An engine that loses its coolant flow runs hot fast, and a hot engine can warp a head, so it needs the pump changed.
The diagnosis
A pressure check and a look at the cooling system traced the overheating to the coolant pump, which was failing, weeping from its seal and noisy on its bearing. The radiator, the hoses and the expansion tank checked out, so the pump was the fault. That's a pump replacement, with a fresh seal, and the thermostat in the same circuit done at the same time so it isn't back in here soon.
The work
The cooling system was drained, the failed coolant pump and the thermostat removed, and new genuine Mercedes-spec parts fitted with fresh seals. The system was refilled with the correct coolant, the air bled out the proper way, and held under pressure to confirm the seals were dry. A road test confirmed the gauge sat steady and there was nothing dripping.
The outcome
Gauge steady through traffic and at speed, no steam, no coolant under the car, no whine, and the level holding. The A200 went home with the cooling system back to spec. A failed coolant pump only stays failed, and the failure at the end is an overheat that can cost a head gasket, so changing the pump and the thermostat together put the whole circuit right in one visit.