How to test and change an ignition coil
The ignition coil plays a crucial role in getting your car started and keeping the engine running smoothly once you’re on the road. Without it, your ignition system wouldn’t be able to generate a spark, meaning the fuel in the engine combustion chamber wouldn’t ignite. And you won’t get very far if your car can’t burn fuel.
If you suspect there’s an issue with the ignition coil, you’re going to want to test and check it as soon as possible, to see if it needs replacing. In this guide, we’ll explain everything you need to know about the ignition coil, including:
- What is the ignition coil?
- Symptoms of a bad coil
- How to test an ignition coil
- How to change an ignition coil
Every time you turn the key or hit the ignition button, a complex chain of events unfolds inside the ignition system in order to get your car moving. The ignition coil is a crucial part of this system. It works by transforming the 12-volt current (also called the low-tension current) stored in the car battery into a powerful high-tension current — sometimes as much as 45,000 volts. This powerful charge is transmitted out of the ignition coil and through the spark plugs, which generate a spark at exactly the right moment to ignite the mixture of air and fuel in the combustion chamber.
An ignition coil is essentially a wire wound transformer coiled around an insulator. However, there are a number of different kinds of ignition coil, and they come in all sorts of shapes and designs:
- Distributor coils: This type of ignition coil uses a mechanically driven distributor mechanism. It’s usually found on older vehicles and was very common until the 1990s.
- Pencil coils/coil-on-plug ignition coils: This type of coil is mounted right on top of the spark plug. This means the high-voltage pulse is generated directly at spark plug, without the need for cables, so there's no loss of voltage. It’s also a very compact design and doesn't take up much space in the engine bay, which is why they're commonly used in modern cars.
- Ignition coil packs: Coil packs have multiple pencil coils mounted within a single, compact unit known as a "rail". This is then fitted across a row of several spark plugs.
- Can ignition coils: Very old and vintage vehicles often have canister-type ignition coils. Some versions are even filled with oil, which acts as an insulator and helps cool the coil, while others use a more modern solid insulator. They're usually found in cars with breaker-type ignition systems.
You can find out which one is right for your car by checking the owner's manual or contacting the manufacturer.
How exactly does the ignition coil work?
The ignition coil works using the electrical principle of a step-up transformer, using two wires coiled around a laminated iron core. These wires are called the primary and secondary. The secondary wire has lots more coils than the primary — often as many as thousands more, with a winding ratio of around 1:150 to 1:200 primary windings to secondary ones. This disparity between the two wires is key, because it determines the level of voltage that is eventually output.
As we’ve mentioned, the ignition coil is responsible for transforming a low-tension current into a high-tension one. When you turn the ignition, the primary wire receives the 12-volt current from the battery — this is the low-tension current. This generates a magnetic field.
The flow is then intentionally disrupted at the right moment by the car’s engine control unit, causing the magnetic field to collapse suddenly. This in turn generates a high-voltage pulse, which travels along the secondary wire and out into the spark plug (usually via thick, insulated wires, although sometimes the plug is directly attached). The plugs then create the spark needed to fire up the engine.
When this important component fails, it can cause serious problems. At first, you might start to notice that the engine misfires or stalls, or that your vehicle’s fuel efficiency has greatly reduced. In severe cases, your car might not start at all.
There are lots of reasons why an ignition coil might fail. For one thing, they go through a lot in the engine bay, which gets incredibly hot when the engine is running, and rattles and shakes a lot when you're out on the road. During the winter, they may also face sudden temperature fluctuations when it gets very cold, which puts additional strain on the ignition coil. All of these factors can weaken or even break the wires and insulation inside the ignition coil. The coil is also very susceptible to moisture damage, which can seep in if the case becomes cracked.
If the ignition coil develops a problem because of the reasons we’ve just outlined, you might notice the following symptoms:
- The engine misfires: When the ignition coil is struggling, it may backfire or misfire repeatedly, especially in damp weather, early in the morning, or during very hot or cold spells of weather.
- Difficulty starting the car: In some cases, the car might not start at all. However, there are lots of other reasons this could be happening — our car guide to troubleshooting car-starting problems should help you diagnose the issue. Ignition issues can also stop your car from starting properly, so it may be the case that you need to replace the distributor, as well.
- Reduced fuel efficiency: When the ignition system is not working properly, the spark plugs cannot create a spark at exactly the right time to burn the fuel in the engine efficiently. This means, if the ignition coil isn’t working as it should, you might find you’re getting fewer miles to the gallon than normal.
While the symptoms we've shared above could all be signs of a bad ignition coil, it’s not necessarily a guarantee that this is the cause of the problem, as another part of the ignition system or engine could be to blame. So, you’ll want to test the function of the ignition coil to confirm that it's defective before you carry out any repairs.
Before you do this, it's also a good idea to inspect the spark plugs, as the symptoms of a bad spark plug and a faulty ignition coil can be remarkably similar. You can read more about how to test them in our dedicated spark plug guide.
To test the ignition coil, you’ll need:
- A voltmeter or digital multimeter
- A set of basic hand tools, including a wrench, screwdriver set, socket set, and ratchet
- Your car owner's handbook or Haynes manual
1. Prep the car and get ready
Start by parking your car on an even surface and engaging the handbrake. Wait for the engine to cool down fully, then disconnect the negative battery cable. This will ensure you don't accidentally ground the circuit and shock yourself.
2. Find the resistance
Next, check your car owner's handbook to find the correct resistance reading for your car's ignition coil. This is measured in ohms (indicated by the symbol) and is usually specified as a range rather than an exact figure.
3. Locate the coil
Locate the ignition coil. Exactly where this will be depends on the make and model of car and the type of coil, but it’s normally located right above the engine or mounted on top of the spark plugs. Refer to the owner's handbook if you need help with this.
4. Disconnect the coil
Remove any plastic covers or harnessing which may be covering the ignition coils. The ignition coil doesn't normally have many mounting bolts — usually just one or two holding the unit in place. Remove the bolts using your hand tools and remove each coil.
5. Test the primary wire
Locate the primary wire. This is normally made of heavy wire and receives energy from the battery. Connect the multimeter's positive and negative leads to the corresponding terminals on the coil. Take the reading on the multimeter and compare it to the recommended resistance reading for the primary winding given in your owner's manual. The resistance on the primary is normally around 0.4 to 2 ohms for most cars, but it can vary from vehicle to vehicle.
If the reading doesn't fall within the correct range, it's defective and the coil should be replaced.
6. Test the secondary wire
Next, test the secondary wire. This will be thinner than the primary winding, with many more coils. Connect the multimeter to the positive terminal and the high-output terminal that connects to the spark plug. Take a reading and compare it with the resistance stated in the handbook. For most cars, this will around 6,000 to 10,000 ohms.
Again, if the reading doesn't match up, you'll need to replace the entire ignition coil.
7. Repeat until all coils in the unit have been tested
If your coil has several coils inside one unit, you'll need to keep repeating steps 5 and 6 until all the primary and secondary windings in the coils have been tested.
Step 8: Finish up
Remove the multimeter. If all the resistance readings were within the specified range, and you can't see any visible signs of damage or wear and tear, you can re-install the coils and replace the mounting bolts. Remember to re-attach the negative battery cable before you close the bonnet.
If the testing revealed the coils to be faulty, you can take out the defective coil and fit a replacement. Skip to step 4 of the section below to find out how to do this. If the coils seem fine but you're still having trouble with the ignition system, you may want to take your car to a professional.
The windings within the ignition coil cannot be repaired, so if you’ve discovered that one or both are faulty, your only option is to change the ignition coil. Fortunately, once you’ve sourced the correct part for your car, replacing the ignition coil is fairly straightforward with most makes and models. Assuming the coil is easy to access, anyone with an intermediate level of knowledge and experience with DIY repairs should be able to change an ignition coil.
Before you start work, gather the following tools and equipment:
- A replacement ignition coil. The correct type of coil for your vehicle should be specified in the owner's handbook. You can also use the handy product-finding tool on our ignition coil page to find the right sort for your car.
- Something to label loose cables with, like coloured tape or marking pens. This isn't essential, but it can make the job a bit easier.
- Protective gloves.
- A set of hand tools, including a wrench, screwdriver set, socket set, and ratchet.
- The owner's handbook. It may also help to have the relevant Haynes manual for your car, although this is not essential.
1. Prepare the car
Park the car on a level surface and engage the handbrake to keep it steady. Then, disconnect the negative battery terminal.
Tip: You should always disconnect the negative battery cable during any of your car's electrical systems. This will ensure you don't accidentally ground the circuit, which can give you a nasty shock and potentially damage your car.
2. Locate the coil
Find the ignition coil inside the engine bay. The owner's handbook can help you with this if you're unsure.
3. Disconnect and remove the coil
Find the mounting bolts and screws that attach the coil to the vehicle. Using your hand tools, remove each of the bolts and set aside.
You will also need to disconnect the electrical connectors that link the coil to the car — this will likely need to be done in a particular order, so refer to the handbook to make sure you're doing it correctly. You'll need to make sure that the wires are attached to the corresponding terminals on the new ignition coil, so it may help to label each wire using some tape or a marker pen to help you keep track of which is which.
The ignition coil should pop off easily once these are removed. Set it to one side.
4. Fit the replacement ignition coil
Install the new ignition coil. Reconnect the electrical cables first — if there are multiple cables, be very careful to make sure you connect each one to the corresponding terminal on the new coil. Replace the mounting bolts or screws last and tighten them. There's no need to overtighten them, as this could damage or break them. They only need to be tight enough to keep the ignition coil securely attached.
5. Check your work
Once the new coil is fitted, you'll want to test it using a multimeter to make sure it's working correctly. To do this, follow the method for testing the ignition coil outlined above.
If everything seems to be working well, you can re-connect the negative battery cable and lower the hood. Then, try turning the engine on. If the engine starts normally, you can go for a test drive.
If the engine fails to start or takes a few tries to turn over, you may want to check that the electrical connections between the ignition coil and the car are sound. If the engine still does not work after this, you will need to take your car to a professional mechanic for further testing.
A faulty ignition coil can wreak havoc with your car's ignition system, and it can even prevent your car from starting at all. So, if you're having an issue, be sure to test and replace it as soon as you can. Remember, we stock a huge selection of replacement ignition coils to suit all makes and models of car, all with free UK delivery when you spend over £20. You can also take advantage of our click-and-collect service, which allows you to pick up your new ignition coil from one of over 70 branches nationwide.
If you’re looking to carry out other repairs on your car, visit our automotive knowledge hub to discover more in-depth guides and resources that will help you keep every part of your vehicle in top condition, all while saving money on repairs.