The brief
The Sharan had been losing coolant fast, fast enough that the owner was topping it up every few days. It's the family MPV that does the daily school run, and he didn't want to gamble on whether the cooling system would hold up on the next trip, so he brought it straight in.
Coolant doesn't disappear on its own, so a level that drops every few days means it's leaking somewhere. On this engine the thermostat sits in a plastic housing that's part of the same module as the water pump, and that plastic ages and cracks, which lets coolant weep out. Run it long enough and you don't lose coolant gently any more, you lose it all at once on the motorway, and an engine that runs out of coolant overheats.
The diagnosis
A pressure test traced the leak to a hairline crack in the plastic thermostat housing, a known ageing failure on this engine. The thermostat in it had also lost a bit of travel, which was making the warm-up inconsistent.
Since the housing, the thermostat and the water pump on this engine are all one integrated module, the sensible fix is to change the assembly complete rather than try to patch a cracked plastic part, so that's what got done.
The work
The cooling system was drained, the old cracked module removed, and a new VAG-spec assembly fitted, water pump and thermostat housing together, with fresh seals.
Then 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 warm-up was back to normal and there was nothing weeping.
The outcome
Warm-up time back to normal, no coolant traces around the housing, the level holding, and the gauge sitting steady in the middle.
The Sharan went back into family duty with the cooling system sealed. A cracked plastic housing only cracks further, and the failure at the end is an overheat that can cost a head gasket, so changing the module before it let go kept it to a clean, planned job.