Coil Pack Failure Symptoms and What It Costs to Fix

Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent

Ignition coils turn the 12 volts from the battery into the very high voltage a spark plug needs. Most modern petrol engines have one coil per cylinder sitting on top of the plug. When one starts to fail, that cylinder stops firing properly and you feel it straight away.

Coil pack failure symptoms

• Misfire: a stutter, jerk or shudder, often worst when pulling away or accelerating uphill
• Rough idle: the car shakes at traffic lights more than usual
• Engine management light: steady for a minor misfire, flashing for a severe one
• Loss of power, as if the engine is running on fewer cylinders
• Poor fuel economy
• Hard starting, especially on cold, damp mornings
• A fuel smell from the exhaust, as unburnt fuel goes through

Coil faults often get worse when the engine is hot or under load, then seem to clear up. That on-off pattern is a typical clue. Our guide to engine misfire symptoms covers other causes that feel similar.

Why the engine light flashing matters

A flashing engine management light means a misfire bad enough to damage the catalytic converter. Unburnt fuel goes into the exhaust and burns inside the cat, overheating it. If the light flashes, ease off, get somewhere safe and stop. Driving on can turn a coil job into a much more expensive exhaust repair.

How a faulty coil is diagnosed

1. Read the codes. Misfire codes often name the cylinder, such as a misfire on cylinder 2. Some cars also log a coil circuit fault.
2. Check the live data. Many cars count misfires per cylinder, which shows exactly where the problem is.
3. Swap test. Move the suspect coil to another cylinder. If the misfire follows the coil, the coil is faulty. If it stays, look at the plug, injector or compression.
4. Inspect the plug wells. Look for water, oil from a leaking rocker cover gasket, or cracked coil boots.
5. Check the spark plugs. Worn or fouled plugs are a common root cause.

This is why guessing and buying a coil online does not always work. A misfire can also come from a plug, an injector, a vacuum leak or a mechanical problem.

What affects the cost to fix

We do not quote a set price without knowing the car, because these things change it a lot:

• Coil design: some engines have individual coils, so only the faulty one needs replacing. Others use a single pack covering all cylinders.
• Access: on many four cylinder engines the coils are right on top. On some V6 engines the rear coils sit under the intake manifold, which adds labour.
• Part price and brand for your make and model
• Spark plugs: if they are due, doing them at the same time saves labour later
• Underlying cause: oil or water in the plug wells needs fixing too, or the new coil will fail

On many cars this is a fairly short job once diagnosed. Send us the reg and we will quote parts and labour.

Can you prevent coil failure?

• Change spark plugs at the interval in the service schedule
• Fix oil leaks from the rocker cover gasket before oil reaches the plug wells
• Keep the scuttle drains below the windscreen clear so rain cannot pool on the engine
• Deal with misfires early rather than driving on them

Damp autumn and winter weather tends to find out weak coils and cracked boots, so a misfire that starts in November is not unusual. If yours is also slow to start, see car won't start in cold weather.

How we can help

We carry out mobile diagnostics with a code reader at your home or workplace across East Kent. We find which cylinder is misfiring and why, replace coil packs and spark plugs, and clear and recheck the codes after a test drive. Labour is £30 to £50 per hour depending on the job, and we quote before any work starts. Message your reg and postcode on WhatsApp 07563 350335 or use our contact page. You can also read about our mobile car diagnostics.

Frequently asked questions

Can I drive with a bad coil pack?

Short distances gently, if the engine light is steady and the car drives reasonably. If the light is flashing or the car is shaking badly, stop, as unburnt fuel can overheat and damage the catalytic converter.

How do you know which coil pack is faulty?

A code reader usually shows a misfire code for a specific cylinder. Swapping that coil with a neighbouring one and seeing if the misfire moves confirms it.

Should spark plugs be changed with coil packs?

Often yes, especially if the plugs are near or past their interval. A worn plug makes a coil work harder and can cause the new coil to fail early.

Why do coil packs fail more in wet weather?

Damp can get into cracked coil boots or plug wells, letting the spark leak to earth. Water pooling in plug wells, for example from a blocked scuttle drain, is a common cause.