The brief
This A5 came in with the power suddenly down and the engine misfiring, the kind of fault that doesn't creep up, it just happens. We ran the Audi scan tool on it, because a sudden power loss with a misfire on a direct-injection engine has a short list of likely causes and the fuel pressure data tells you which one. These engines run a high-pressure fuel pump on the engine, driven off a lobe on the camshaft through a little cam follower. The follower wears, and once it wears through, the pump can't make its pressure, so the injectors run short, the engine misfires, loses power, gets hard to start, and throws a fuel-pressure code. A worn pump and follower don't recover, and if the follower has worn right through it can score the camshaft lobe, so the camshaft gets checked too.
The diagnosis
The Audi scan tool showed a fuel-pressure fault, the actual rail pressure well below target, and a check found the high-pressure fuel pump worn with the cam follower worn down, which is the sudden power loss and the misfire. The camshaft lobe was checked and was still good, no scoring, the low-pressure side and the injectors were fine. That's a high-pressure fuel pump and cam follower replacement, rather than chasing a misfire that's really a fuelling problem.
The work
The old high-pressure fuel pump was removed, the worn cam follower replaced, the camshaft lobe checked clean, and a new genuine Audi-spec high-pressure pump fitted, the connections checked tight and the fuel system bled and pressurised. The fault was cleared and the adaptations reset so the engine relearns on proper fuel pressure. A road test confirmed a clean start, no misfire, the rail pressure holding target, and full power back.
The outcome
Full power back, a steady idle, no misfire, no warning light, easy starting, and the fuel pressure holding where it should. The A5 went home running properly again. A worn cam follower only wears further and eventually takes the camshaft with it, so catching it and replacing the pump and follower fixed it before it became a much bigger job.
