How to test an ignition coil

Are you experiencing problems starting your car or finding your engine is misfiring? It could be a faulty ignition coil. Your car’s ignition coil transforms the low voltage charge provided by your battery to the high voltage needed to power the spark plugs, ignite the fuel and keep your car’s engine firing on all cylinders.

Your car’s ignition coil plays a crucial role at the heart of a complex ignition system. In this guide, we'll explain what the ignition coil is, what it does, and how you can check that it is working correctly.

Ignition coils are effectively step-up transformers, changing the small charge from your car's 12-volt battery to tens of thousands of volts, which is enough to operate the spark plug.

Inside the coil are two separate wire windings coiled around a central iron core. The primary winding is much shorter than the secondary winding, which is much longer. This matters because the more windings, the more voltage the coil can produce.

When you turn the key in the ignition, the battery sends a current through the primary wire, which creates a magnetic field. The engine control unit (ECU) cuts this charge off at precisely the right moment, causing the field to collapse, which generates a high-volume current which travels through the secondary winding to the spark plug.

This all happens thousands of times a minute to keep your engine running smoothly. In modern vehicles, you’ll generally find one ignition coil per cylinder/spark plug. So, a 4-cylinder car would have 4 coils, 6 cylinders, 6 coils and so on.

In most cars, the ignition coil is directly connected to the spark plugs. You will be able to find the ignition coil in your car by opening the bonnet and looking at the top of the engine.

There are a variety of ignition coil types, including:

• Single coils, typically fitted to older vehicles (pre 2000)
• Pencil coils that look like a long and thin tube mounted on the top of the spark plugs
• Ignition coil packs contain several pencil coils in a single unit
• Can type ignition coils, usually found fitted to classic vehicles with breaker-type ignition systems

Most modern cars will use pencil coils or an ignition coil pack, but to be sure, check your car’s maintenance manual or search online for more information.

Some of the warning signs your ignition coil is faulty or on the way out include:

• Engine misfiring: If a coil fails, your engine won’t be running on all cylinders. You could find your car misfires, especially during damp weather.
• Problems starting the car: If your car doesn't start, it could be a huge range of issues – including the ignition coil.
• Poor fuel efficiency: If the ignition coil isn't powering your spark plugs, this can lead to fuel wastage (and bigger fuel bills!).

Testing the ignition coil is a fairly simple process. The tools you need include:

• Digital multimeter
• Wrench
• Socket set
• Screwdriver
• Gloves and work clothes
• Car owner’s manual

When working on your car, it's always worth having a camera handy to take pictures as you work. Taking photos while you work on disassembling and fixing your care can help you reassemble things in the correct order and ensure everything is in the right place.

Park your car somewhere safe and let the engine cool down. Remove the negative battery cable (the one with a black cap) to avoid a nasty shock.

You'll need to get a copy of your car's handbook (or search online) to find the correct resistance rating for your coil. It'll be in ohms and an operating range rather than a single figure.

The coil will be located right above the spark plugs. When you’ve found it, disconnect it from the spark plugs. You should be able to do this by hand, but if the coil is secured in place with bolts, then remove them (and put them somewhere safe).

First, select Ohms on your multimeter. Connect your digital multimeter’s positive and negative leads to the smaller outer terminals on the coil. This is the primary winding. You can now take a resistance reading on the multimeter and compare it to the values in the handbook.

While all ignition coils are different, typical values range between 0.4 – 2 ohms for the primary wire.

Leave one of your leads on one of the outer terminals and connect the other to the larger centre terminal and take a reading. This is the secondary winding. Typical values range between 6,000 – 10,000 ohms (but check your car’s workshop manual for an accurate figure).

You need to repeat the process for each ignition coil, testing the primary and secondary windings to check whether they're in the correct operating range.

If the readings are all fine, then your ignition coil looks to be working fine and the problem is elsewhere. If you identify a problem, then you’ll need to replace the ignition coil. Even if you quickly identify one faulty coil, we recommend testing them all to check they’re all working correctly.

If you need to replace your ignition coil, you’ll find everything you need at Halfords. We stock a massive range of replacement ignition parts for most cars. Type in your registration and you’ll see the correct parts for your vehicle, including ignition coils and spark plugs. You can also find the tools you need, including accurate digital multimeters required for resistance testing.

If you're experiencing an ignition problem but don't have the tools or time to diagnose it, book an appointment at Halfords Autocentre. During your appointment, one of our trained technicians will help you. You'll be on the road and ready for the journey ahead in no time.