P2302: Ignition Coil A Secondary Circuit
Do not keep driving
Ask when the spark plugs were last changed before agreeing to a coil. Plug gaps widen as they wear, a wider gap needs more voltage, and eventually the coil cannot deliver it — so the coil reports a fault while the plug is the part that is finished. That is the single most common way money gets wasted on this code, and a set of plugs is far cheaper than a set of coils. Stop driving if the engine light is flashing. Worth mentioning if the misfire is worse in wet weather, which points at the coil boot or a crack in the coil body rather than the plug.
What this code means
A fault on the high-voltage output side of coil A. This is the code where the coil most often gets replaced and the spark plug was the problem.
What the secondary circuit has to do. The coil's job is to produce whatever voltage is needed to push a spark across the plug gap. That figure is not fixed — it is set by the conditions in the cylinder and by the gap itself, and the coil produces what is demanded up to its limit.
Spark plug gaps widen as they wear, because every spark removes a small amount of metal from the electrodes. A wider gap needs more voltage. So the demand climbs steadily across the plug's life, and at some point it exceeds what the coil can deliver — at which point the spark fails, and the coil reports a secondary fault.
The coil is doing exactly what it was designed to do. It is being asked for more than it can give. Which is why:
- Replacing the coil often fixes it briefly, because a new coil has slightly more margin — and then the fault returns as the plug wears a little further.
- Coils get replaced in sets on cars that needed a set of spark plugs.
- The cheap check comes first: how old are the plugs, and how many miles have they done? Modern intervals are long enough — often sixty to a hundred thousand miles — that people genuinely forget plugs are a service item at all.
Anything else that raises demand voltage does the same thing, and is worth knowing because it explains a secondary fault on plugs that are not worn: a lean mixture, high cylinder pressure on a boosted engine, or a plug of the wrong heat range or gap fitted at a previous service.
The genuinely secondary-side failures — a cracked coil body tracking to earth, a perished boot, or a carbon track in the plug well — are real but are the second thing to look for, not the first. They also tend to be visible, and they are worse in the wet.
Likely causes
Start here
- Worn or fouled spark plugs · easy
- Incorrect spark plug gap or heat range · easy
- Faulty ignition coil · easy
- Perished or tracking coil boot · easy
Less common
- Cracked coil body tracking to earth · easy
- Lean mixture raising demand voltage · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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