The brief
The A6 had been triggering its coolant top-up warning regularly. The owner kept refilling, the warning would go away for a while, then come back. He had also spotted a damp patch under the car around the radiator core area, and brought it in.
A warning that keeps coming back after a top-up is the system telling you the loss is ongoing, not a one-off. And a damp patch localised to the radiator area is a strong hint at where the loss is. Time to find it properly rather than keep feeding the system.
The diagnosis
A pressurised cooling-system test traced the leak to a hairline crack along the plastic end-tank seam of the radiator. That is a classic ageing failure: the plastic side tanks on these radiators get brittle with years and heat cycles, and a fatigue crack opens up at the seam where the plastic meets the core.
We checked the rest of the loop, since a coolant leak can come from several places. The hoses, the water pump, the expansion tank and the thermostat housing all checked out clean. The radiator was the failed part, and there is no repair worth doing on a cracked plastic end tank. Replacement was the only proper fix.
With the front end coming apart for the radiator anyway, the connecting coolant hoses got done at the same time, since they were the same age and old hoses are a leak waiting to happen.
The work
Drained the system, removed the radiator hoses, and pulled the front of the car apart far enough to lift the failed radiator out, which on this car means removing the bumper and the radiator support.
Fitted a new Audi-spec radiator, reseated all the hoses with fresh clamps, fitted the new coolant hoses, and refilled with the correct coolant at the right ratio.
Then the bleed cycle, run the proper way, because a modern cooling system traps air pockets after a refill that topping up alone will not clear. Once the system was purged and up to temperature, we held pressure on it to confirm a sealed result before the car went out.
The outcome
No drips. Coolant level holding steady. No warning.
The A6 went home with the cooling system back to spec and the persistent top-up warning silenced for good. For the owner, that means no more refilling the system every couple of weeks, no more warning light, and a car that runs at the right temperature.
And with a cracked radiator caught at the slow-leak stage rather than allowed to fail completely, the engine was never at risk of an overheating event, which is the expensive way this story ends if a coolant leak is ignored.
