The brief
The A5 kept needing coolant topped up between services, the level dropping with a faint sweet smell and a bit of weeping around the reservoir. He brought it in, which is exactly right, coolant that disappears is going somewhere and a slow leak turns into an overheat the day it lets go. When one of these loses coolant slowly, the expansion tank is the usual culprit. The expansion tank is the plastic reservoir that holds the spare coolant and lets the system breathe as it heats and cools, and it sits under pressure and heat cycles its whole life. The plastic gets brittle with age and cracks, often at a seam or around the cap neck or the level sensor, and it weeps coolant that you smell more than see. A cracked tank only splits worse, so it needs replacing.
The diagnosis
A pressure test on the cooling system pinpointed it, the expansion tank was weeping from a hairline crack and losing pressure slowly, which is the disappearing coolant. The radiator, the hoses, the water pump and the rest of the system held fine. That's a tank replacement, with a fresh cap, rather than chasing a crack in brittle plastic that's only going to spread.
The work
The cooling system was drained enough to get at the tank, the old cracked expansion tank removed, and a new genuine Audi-spec tank fitted with a fresh cap and the level sensor transferred over properly. The system was refilled with the correct Audi coolant, bled the proper way so no air pockets were left, and pressure tested again to confirm it held with no weep. A road test confirmed the gauge sat steady and the level stayed put.
The outcome
No more coolant loss, the level holding between checks, the gauge steady, and the system holding pressure as it should. The A5 went home with the leak resolved. A cracked expansion tank only splits further, and the failure at the end is a sudden coolant dump and an overheat, so changing the tank kept it to a tidy, planned job.
