The brief
Benny's A4 was getting through coolant fast, the level dropping quickly between checks, so he was referred to us to find the leak. Coolant disappearing at that rate is going somewhere, and a leak that's getting worse turns into an overheat soon, which can take the engine with it, so it needed pinning down. A common coolant leak on these is the hose that runs to the expansion tank, the reservoir that holds the system's spare coolant. That hose carries hot coolant under pressure for years until the rubber hardens and cracks, usually at a bend or where it clamps onto a stub, and once it splits it weeps under pressure and the level drops. A perished hose doesn't reseal, so it gets replaced with fresh clamps.
The diagnosis
A pressure test on the cooling system brought the leak out, the hose to the expansion tank was perished and weeping at a crack, which is the fast-dropping coolant. The radiator, the water pump, the tank itself and the rest of the system held fine. That's a hose replacement with new clamps, rather than chasing a split in hardened rubber that's only going to spread.
The work
The cooling system was drained enough to get at it, the old perished hose removed and a new genuine Audi-spec hose fitted with new clamps and the connections checked. 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. The other hoses got a look while it was apart. 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. The A4 went home with the leak resolved. A perished coolant hose only splits further, and the failure at the end is a sudden coolant dump and an overheat, so the pressure test that found it and the new hose kept it to a tidy job.