The brief
The A4's idle had grown uneven enough that the owner could feel it through the steering wheel at the lights. Fuel economy had been creeping down on his regular routes. Cold cranks were taking longer than they used to. And a check engine light was flagging two cylinders.
That cluster, rough idle plus worse economy plus slow cold cranking plus a multi-cylinder misfire code, is the classic signature of an ignition system that has aged out. Worn plugs and tired coils between them make every one of those symptoms. Time for a proper look rather than a guess.
The diagnosis
We pulled the plugs first. The mileage was about 70,000 km past the last set, so they were due regardless of what the codes said: a plug that has done 70,000 km has a wide-open gap and a rounded electrode, and it makes a weaker spark.
Then the coils. Two of the four tested high secondary resistance on the bench, which is the electrical signature of a coil that can no longer build a full-strength spark, and that was what was producing the cylinder-specific misfires the codes had flagged. With the plugs at 70,000 km and the coils past 130,000 km, both were due. The right call was the full set of each, not picking off the two failing coils and waiting for the others.
The work
Fitted four new VAG-spec spark plugs, each gapped to the workshop manual figure and torqued in carefully. Plugs do not forgive over-tightening, so each one gets the manufacturer's torque, no more.
Replaced all four ignition coils with new VAG-spec units, reseated every harness clip, and cleared the stored fault codes on the scanner. Then took the car out for a road test through a full drive cycle, watching the live misfire counters.
The outcome
Idle smoothed out, no more buzz through the steering wheel at the lights. Fuel economy back to normal across the next tank. Normal cold cranking. No check engine light return after a full drive cycle.
The A4 went home with the ignition system reset. For the owner, that means a smooth-running engine, a fuel bill back where it should be, and an ignition setup good for another long stretch, rather than a fault that comes back cylinder by cylinder over the coming year.
Doing the full set of plugs and coils once costs less than doing pieces of it four times.