The ignition coil: a small part, big trouble when it fails
You are driving along, everything is fine, and suddenly the engine starts to stutter. It loses power in bursts, burns more fuel than usual, and the check engine light is happily blinking at you. There is a good chance an ignition coil has failed. It is one of the most common faults on modern engines and, thankfully, one of the cheaper to fix, especially if you consider a used coil.
But before you dive into buying, let us walk through everything you need to know about ignition coils, how they work, why they fail, and when it makes sense to buy used instead of new.
How an ignition coil works
The working principle of an ignition coil is actually fairly simple, at least on paper. The coil is a transformer that converts the low voltage from the battery (12V) into the high voltage needed to create a spark at the spark plug (20,000 to 40,000 volts).
Inside the coil there are two windings of wire, primary and secondary, wound around an iron core. When the ECU sends a signal, current flows through the primary winding and creates a magnetic field. When the current is suddenly cut, the magnetic field collapses and induces a high voltage in the secondary winding. That high voltage travels to the spark plug and creates the spark that ignites the fuel-air mixture in the cylinder.
Sounds simple, but the coil has to do this hundreds of times per second, at high temperatures and with vibration. That is why they wear out over time.
Types of ignition coils
Several types of coils have been used through the history of cars:
- Classic single coil with a distributor - an older system where one coil feeds all cylinders via a distributor. Rarely seen on newer cars today.
- DIS (Distributorless Ignition System) - uses one coil for two cylinders (waste spark principle). Common on cars from the 90s and early 2000s.
- COP (Coil-On-Plug) - every cylinder has its own coil mounted directly on the spark plug. The most common system on modern cars. The advantage is more precise ignition control and fewer losses.
- CNP (Coil-Near-Plug) - similar to COP, but the coil is not directly on the spark plug but nearby, connected by a short cable.
The type of coil you need depends on your car. COP coils are the most common on modern vehicles and the easiest to replace, you simply pull the old one out and plug the new one in.
Symptoms of a bad ignition coil
When a coil starts failing, the symptoms are usually pretty obvious:
Engine misfire - this is the most obvious symptom. The engine does not run smoothly, you feel hesitation, especially under acceleration or at idle. On a COP system, usually only one cylinder misfires, so the engine is rough but still drivable.
Loss of power - when one cylinder stops working, the engine loses about 25% of its power (on a four-cylinder). That is noticeable, especially under acceleration and on hills.
Higher fuel consumption - unburned fuel from the misfiring cylinder goes into the exhaust. Apart from being a waste of money, it can damage the catalytic converter, which is a much more expensive repair.
Check engine light - the ECU detects the misfire and turns the light on. A diagnostic tool usually shows code P0300 (random misfire) or P0301-P0306 (misfire on a specific cylinder).
Hard starting - if multiple coils are weak, the engine can be hard to start, especially in cold weather.
Smell of fuel from the exhaust - unburned fuel comes out through the exhaust. If you smell strong petrol from the tailpipe, that is a serious sign of ignition trouble.
Why coils fail
Ignition coils have no moving parts, so in theory they should last forever. In practice they are killed by:
Heat - coils sit on an engine that produces enormous amounts of heat. Every heating and cooling cycle creates thermal stress that gradually degrades the winding insulation.
Vibration - constant engine vibration can cause micro-cracks in the insulation or weaken internal connections.
Worn spark plugs - this is a common and underrated cause. When the spark plug is worn, the gap between the electrodes is wider, so the coil has to produce a higher voltage for the spark. That puts extra strain on it and shortens its life. So whenever you replace coils, check the plugs too.
Moisture - water getting into the spark plug well can cause a short circuit and damage the coil. This is a common issue on engines where the plugs sit in deep wells.
Age - quite simply, every electronic component has a limited service life. After 8 to 10 years, ignition coils are at the end of their useful life.
When it makes sense to buy a used coil
Honestly, ignition coils are one of the cheaper car parts. A new aftermarket COP coil costs 15-40 euros, and an original 30-80 euros. A used one usually goes for 10-25 euros. The saving on a single coil is not huge.
But here is when a used coil makes sense:
- When you need to replace all coils at once - on a six-cylinder engine, replacing all 6 original coils can cost 300-500 euros. A used set for 60-100 euros is a meaningful saving.
- When the originals are much better than aftermarket - on some engines (BMW N52, N54 for example), the original Bosch or Delphi coils are noticeably better than cheap replacements. A used original beats a cheap new one.
- Older or rare models - for some older cars, new coils are hard to find or unreasonably expensive. Used is the only practical option.
- DIS coils - on older systems with DIS coils, those coils tend to be expensive and used is a logical choice.
How to test a used ignition coil
Testing a coil is fairly straightforward if you have basic tools:
Resistance test with a multimeter
Every coil has a specification for primary and secondary winding resistance. Broadly speaking:
- Primary winding: 0.4 to 2 ohms
- Secondary winding: 5,000 to 15,000 ohms
If the values are outside the range, the coil is bad. Caveat: a coil can pass the resistance test and still be faulty. The resistance test is good at catching completely blown windings but does not catch intermittent faults that only show up under load or at high temperature.
Visual inspection
Look at the connector, it must not be melted or deformed (a sign of overheating). The housing must be crack-free. The rubber boot that goes into the spark plug well must be flexible and undamaged, because if it lets moisture in, the coil will not work reliably.
Practical test
The best test is to fit the coil and see whether the engine runs smoothly on that cylinder. If you have multiple cylinders, you can swap suspect coils between cylinders and see whether the problem follows the coil or stays on the same cylinder.
Coil replacement: tips
Replacing a COP coil is one of the simplest repairs on a car:
- Unplug the electrical connector from the coil
- Undo the bolt holding the coil (usually one)
- Pull the coil out of the spark plug well
- Fit the new coil in reverse order
Literally five minutes of work per coil, all you need is a torx or hex key. If you can do it yourself, you save on labour costs.
One important tip: whenever you replace a coil, always check the condition of the spark plug on that cylinder. If the plug is worn, replace it too. A worn plug will kill the new coil faster and you will be back at square one.
Should I replace all coils at once
This is a common dilemma. The answer depends on the situation:
If only one coil has failed and the others are relatively new (under 60,000 km), you can replace just that one. But if all the coils are original and the car has done 150,000+ km, it is probably smarter to replace them all because the rest will start failing one by one soon.
Replacing them all at once is more economical because you only pay for labour once (on some engines accessing the coils means removing the engine cover and other parts), plus you have peace of mind that they are all the same age and condition.
Where to buy used coils
You can find used ignition coils at scrapyards, online used-parts platforms, and of course on the PoDi platform. The advantage of PoDi is that you can get offers from several suppliers and pick the best option on price and warranty.
When buying used coils, try to buy a set from the same vehicle. That way you know they are all the same age and condition, and there is less chance of trouble. Avoid buying individual coils from various sources because you may end up with coils of different ages and condition.
At the end of the day, used ignition coils are a safe way to save money, especially if you buy original coils from well-known makers like Bosch, Delphi or Denso. Those coils are sturdy and long-lasting, and the price difference between used and new can be significant, especially when you need several of them.