Audi Case Study · 203

Audi Q5 engine misfire, resolved.

Q5 came in misfiring under load, with a check engine light flagging one cylinder. Coil swap-test isolated the failed coil. New coil plus a fresh plug on that cylinder, codes cleared.

Job done

Mechanical Repairs Engine Diagnostics Audi Specialist
Audi Q5 in the workshop for misfire diagnosis.

The brief

The Q5 had a misfire that showed up on heavier acceleration, the kind of stumble you feel when you put your foot down. And the check engine light was on.

The codes had it pinned to cylinder three. The owner brought it in to find the actual cause rather than have someone replace all four coils on a guess, which is the right instinct, even though, as it turned out, the full set was the answer here for a different reason.

The Audi Q5 up on the two-post lift, in to track down the misfire.
The Audi Q5 up on the two-post lift, in to track down the misfire.

The diagnosis

We swap-tested the suspect coil with a healthy one from another cylinder. The misfire followed the coil, which isolated the failed component cleanly: it was that coil, not the cylinder.

But pulling the plug on that cylinder, and then looking at its neighbours, changed the scope. All four coils were the same age, and the plugs were worn, the affected one worse than the others but the rest not far behind. Doing just the one coil and one plug would mean coming back for the next one soon, paying the access labour again.

So the call was a full ignition refresh: all four coils, all four plugs, done together while the area was open.

The spark plug from the misfiring cylinder, the threads discoloured and the electrode worn.
The spark plug from the misfiring cylinder, the threads discoloured and the electrode worn.

The work

Removed all four ignition coils and all four spark plugs. In went new VAG-spec coils and new plugs, the plugs gapped to the workshop manual figure and torqued in carefully.

Reseated every harness clip, since a coil connector that is not fully home will mimic a failing coil and send you chasing a fault that is not there. Then cleared the stored fault codes on the scanner and took the car out for a road test through a full drive cycle, watching the live misfire counters.

The four old spark plugs (left, rusted threads) next to the four new VAG-spec replacements (right).
The four old spark plugs (left, rusted threads) next to the four new VAG-spec replacements (right).

The outcome

No misfires across the road test, even on heavier acceleration. Check engine light cleared and stayed off after a full drive cycle. Idle smoothed back to normal.

The Q5 went home with the misfire resolved. For the owner, a smooth engine that does not stumble under load, and an ignition system refreshed as a complete set, so this is not a fault that comes back cylinder by cylinder over the next year.

Doing it once, properly, costs less in the long run than doing it four times.

The ignition coils, the old set (left) and the new VAG-spec replacements (right). All four went in together.
The ignition coils, the old set (left) and the new VAG-spec replacements (right). All four went in together.
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