Your engine’s juddering at the lights, the dashboard warning light has come on, and the car feels down on power. More often than not, those are faulty coil pack symptoms, and ignoring them can turn a cheap fix into an expensive one. Spotting the signs early is the difference between a small bill and a wrecked catalytic converter.
This guide explains what a coil pack does, the symptoms of a failing one, what causes them to go, whether you can keep driving, and the point where an old car isn’t worth the repair.
What is a coil pack and what does it do?
A coil pack takes the 12 volts from your car battery and steps it up to the tens of thousands of volts needed to fire the spark plugs. Without that high-voltage spark, the fuel in the cylinder can’t ignite, so the engine misfires.
Older cars used a single coil feeding every cylinder through HT leads, whereas most modern cars use a coil-on-plug setup with one coil per spark plug, and a coil pack sits in between as a single unit serving two or more cylinders. You’ll still find them on older Ford, Vauxhall and Renault models across the UK.
What are the symptoms of a faulty coil pack?
Faulty coil pack symptoms tend to arrive together. The classic one is an engine misfire, usually felt as a judder or stumble at idle or under acceleration. Because the coil feeds the spark plugs, a failing one disrupts combustion fast.
The most common warning signs are below.
- Engine misfire, a noticeable stumble, hesitation or shake.
- Rough or lumpy idle, where the engine feels uneven when stationary.
- Engine-management light, often flashing when a misfire is active.
- Loss of power or the car dropping into limp mode.
- Poor fuel economy, as unburnt fuel is wasted.
- Hard starting or stalling, especially when cold.
- Visible damage on the coil itself, such as cracks, corrosion or oil.
An engine-management light that stays on will also fail an MOT, so it’s worth dealing with before your test is due.
What causes a coil pack to fail?
Coil packs fail mostly through heat and age. They sit on a hot engine and go through constant temperature swings, and that wear eventually breaks down the internal windings.
The usual culprits are below.
- Heat and vibration from years of engine cycles.
- High mileage on the original coils.
- Worn spark plugs, which make the coil work harder and overload it.
- Oil or water getting in, contaminating the coil.
Worn plugs are the one people miss. A tired plug raises the voltage the coil has to produce, so replacing plugs and coils together often makes sense.
Can you drive with a faulty coil pack?
You can usually still drive, because most cars cut fuel to the misfiring cylinder, but it isn’t recommended. The risk is that a misfire pushes unburnt fuel into the exhaust, where it can overheat and ruin the catalytic converter.
That’s the trap. A coil pack is cheap. A damaged catalytic converter is not. Driving on a misfire can also dilute the engine oil and put extra strain on the remaining coils. If you have to drive at all, keep it short and gentle, and get it looked at quickly.
How do you test a coil pack?
The quickest check is an OBD-II scan, which reads the misfire fault code and points to the affected cylinder. Codes in the P0300 range cover misfires, with P0301 to P0306 naming the exact cylinder.
From there, a common trick is to swap the suspect coil to another cylinder and rescan, and if the misfire moves with it then the coil is the fault. A multimeter test confirms it. Most garages, though, diagnose it straight from the codes and a swap.
How much does it cost to replace a coil pack?
Replacing a single coil typically costs around £80 to £250 fitted, depending on the car and the part. The coil itself can be as little as £20 for a budget unit, with most of the cost being labour.
If several coils have gone, or the garage suggests doing the full set with spark plugs, the bill climbs. That’s still minor money against the alternative. That’s still minor against catalytic converter damage, which is why a misfire is worth fixing fast rather than driving through.
When is a coil pack not worth fixing?
On a newer car in good shape, a coil pack is a routine, sensible repair. On an older, high-mileage car it’s worth a second thought, because coils tend to fail in sequence rather than alone, and one replacement is often quickly followed by the next as the rest reach the end of their life too.
When a misfire has already damaged the catalytic converter, or the car has a stack of other faults piling up, the combined repair cost can pass what the car is worth. At that point scrapping makes more sense. You can find out what your car is worth with a free valuation, compare it against the repair quote, and if scrapping wins we offer free collection and an instant online quote.
Frequently asked questions about faulty coil packs
How long can you drive with a faulty coil pack?
Only short distances, and it isn’t advised. The car may run, but the misfire risks overheating the catalytic converter and straining other components. Any persistent misfire should be repaired as soon as possible to avoid a far bigger bill.
Can a bad coil pack damage other parts?
Yes. The main risk is the catalytic converter, which can be ruined by unburnt fuel from the misfire. A misfire can also dilute the engine oil and put extra load on the remaining coils, so one fault can lead to several.
What’s the difference between a coil pack and an ignition coil?
An ignition coil is the component that creates the spark. A coil pack is one type, a single unit serving two or more cylinders, common on older cars. Newer cars usually fit a separate coil on top of each spark plug instead.
Will a faulty coil pack fail an MOT?
Indirectly, yes. A coil pack isn’t tested on its own, but a lit engine-management light is an automatic MOT failure, and a misfire can also push the car over the emissions limit. Both mean a fail until the fault is fixed.
Is it worth replacing one coil or all of them?
On a car with individual coils, you can replace just the failed one. But on a high-mileage engine, the others are often close behind, so many owners change the full set with spark plugs to avoid repeat garage visits.
