The brief
The A6 had quietly developed a slightly lazy throttle response, and the idle had picked up a faint roughness. Nothing dramatic, but the owner had noticed, and both are consistent with plugs and coils that have aged.
The mileage put two intervals at the same point: the 60,000 km spark plug interval, and the 120,000 km ignition coil interval. When those line up like that, it makes sense to do them together rather than in two separate visits, because the access labour is the same either way, and you only pay it once.
The diagnosis
We pulled the plugs first. The electrode wear was as expected for the mileage, no fouling, no sign of oil intrusion, just the normal rounding-off and gap-opening of a plug that has done its 60,000 km.
Then the coils, tested individually on the scanner. Two of the four showed high secondary resistance, the kind of soft fail that produces a slightly rough idle and a lazy throttle without quite throwing a misfire code. That is what the owner had been feeling. The right call was to replace the full set of four coils rather than picking off the two soft ones and waiting for the others to follow, because they are all the same age and the others will not be far behind.
The work
Fitted four new VAG-spec spark plugs at the workshop manual gap, each torqued to spec. Plugs do not forgive over-tightening, so each gets the manufacturer's figure.
Replaced all four ignition coils with new VAG-spec units, reseated every harness clip, then ran a brief idle cycle to verify clean firing on every cylinder before the car went out.
The outcome
Idle smoothed out. Throttle response back to where it should be. No fault codes.
The A6 went home with the ignition system reset for another long stretch. For the owner, that means the engine feeling sharp again at idle and on the throttle, and the ignition side handled as a complete set, so this is not something that comes back in pieces over the next year.
Doing the plugs and coils together while the access was already open is the cheaper path, and the smoother one.