Ignition Coil

Ignition Coil – How to Diagnose and Replacement Cost

Your car’s ignition system is responsible for combusting the compressed fuel and air mixture inside your engine’s cylinder. The resulting combustion moves your pistons, which in turn moves your crankshafts which powers the car to move forward. Over time, your ignition coil could go bad, in which case you will need to replace it. We’ll be discussing more on how an ignition coil works, the symptoms of a bad ignition coil, and replacement parts and cost.

How Ignition Coil Works

An ignition coil takes the low voltage power from your car’s battery, and turn them into high voltage electricity. Then they transfer it onto the spark plugs which then creates an electric spark at its tip. The spark then ignites the fuel and air mixture inside of your engine’s cylinders, starting the combustion process. Put it simply, it transfers electricity from your battery to create a spark and start your combustion process.

Unless you have a diesel engine, the ignition coil is essential for your engine’s operation. If it were to go bad, your car won’t run properly or won’t even start. A diesel engine on the other hand simply relies on its high compression within the cylinders to start the combustion process, eliminating the need for spark plugs and ignition coils. This is part of the reason why diesel engines often run more reliably, as they don’t need ignition coils and spark plugs to run. So if you have a diesel car, you can rest easy.

You can learn more about ignition systems in the video below:

Types of Ignition Coils

The principles for all ignition coil types are the same: they take electricity, feed it to the spark plugs, and your combustion process starts. However, there are several types of ignition coils you should know about.

Can-type ignition coil

If your car was made before 1990, chances are this is the type of coil that you have. Called can-type because of their cylindrical shape, the can-type ignition coil connects to your spark plugs and transfers electricity via high voltage or HT cables and the distributor. Older versions would have been filled with oil which acts as an insulator and a coolant. However, most of them have a dry insulation design.

Can-type is largely similar to distributor coils as they distribute the high voltage power from one coil to all the spark plugs. This segues us perfectly to the next one:

Distributor coil

In principle, distributor coils are similar to can-type as they both distribute high voltage power from one coil to multiple spark plugs. The difference is that in a distributor coil they are mechanically driven.

Ignition block coil

An ignition block is somewhat similar to distributor-type coils. However, instead of having just one coil, they have several coils inside them depending on how many the car needs. They are then connected by HT cables to each spark plug.

Ignition blocks are available with either a single or dual spark technology. A single-spark ignition block would supply each cylinder with high voltage electricity. Meanwhile, in a dual-spark block, the high voltage electricity is fed simultaneously to two cylinders. In this case, one of the cylinders has to be on the “power stroke” and the other on the “exhaust stroke”.

Pencil or coil-on-plug ignition coil

How to Diagnose and Replacement Cost

The previously-mentioned type of coils normally sits remotely from the engine. On the other hand, a pencil or coil-on-plug ignition coil sits on top of the spark plug, hence the coil-on-plug name. It’s also often called a pencil coil because of the shape that resembles a pencil.

Anyway, since they sit right on top of the spark plugs, they can feed the voltage straight into the plugs, which minimizes power loss. They are used in vehicles with an electronic ignition system, and much like the ignition block coil, they are available as either single-spark or dual-spark coils. Most modern cars would use this type of coil as it’s more efficient.

Ignition coil pack systems

A coil pack system is simply a collection of coil-on-plug system housed together in one component, hence the name “coil pack”. They’re also often known as “rail”. The rail is placed across a bank of spark plugs, and they work the same way as a coil-on-plug system. The only difference being they are housed within one component.

Symptoms of a Bad Ignition Coil

Since ignition coils are essential to your car’s operation, there are obvious signs when it has gone bad. Here are the common signs of a bad ignition coil:

Check engine light

Ignition coil testing

A check engine light can come on because of a variety of reasons, faulty ignition coils included. If you have an OBD reader, you can try plugging it in to see what’s the problem. Most cars will have an error code for a faulty ignition coil and you should be able to read if there’s a problem with your ignition coil or if it’s something else entirely.

If you don’t have an OBD reader, you can borrow it from a friend or a mechanic that you know. Or you can simply take your car to a repair shop and ask to do a diagnosis. In which case will normally cost you around $40 – $75 depending on your car’s make and model.

Performance problems

Since ignition coils are essential to your engine’s combustion process, it’s common to encounter performance problems when your car has a faulty ignition coil. One of the most common symptoms is your engine won’t run on all cylinders, sometimes known as “limping”. If your car has multiple coils and only one of them has gone bad, then the cylinder with the faulty coil won’t run. This is not to be confused with “limp mode”, which is a self-preservation mode in a car where it limits performance due to transmission problems.

If you have a distributor system, this can also happen because of a bad HT cable. You may experience loss of power while driving as the car is not running on all cylinders. It’s also possible that your car will stall when you bring it to a halt.

Your car won’t start

Ignition Coil testing not starting

There are many reasons why your car won’t start, such as a flat battery, a broken starter motor, and yes, ignition coils included. If your car has a single-coil such as with the can-type coil, then it’s possible the car won’t start when the coil goes bad. The reason is that all of your spark plugs rely on that one ignition coil to supply power.

To do an initial diagnosis, see if your starter motor still runs when you turn the key. If it does, it’s possible the problem is either your spark plug or ignition coil and not the battery or the starter motor. However, you will still need to do a further diagnosis to determine if it really is your ignition coil. We will discuss this further later on.

Decreased fuel economy

When your ignition coil fails, the spark plugs won’t receive enough electricity to create the necessary spark. Your car then would compensate by transferring more fuel into the cylinders. As a result, you use more fuel and decreasing your fuel economy. While increased fuel consumption can be attributed to a variety of problems, your ignition coil, and spark plugs is some of the easiest ones to check first.

Black smoke and backfiring

Black smoke coming out of your exhaust and backfiring is often the result of too much fuel being injected into your cylinders. Which, as we mentioned above, is a sign of a faulty ignition coil. Your engine can only burn so much fuel at a time, when the cylinders receive too much fuel, the unburned fuel results in backfiring and black smoke.

Of course, there can be a variety of reasons for this, such as faulty fuel injectors. But whatever the case, you should have your car checked if it’s backfiring or emitting black smoke. If left unfixed, it can cause damage to your catalytic converters, leading to more expensive repairs.

How to Diagnose a Bad Ignition Coil

There are several ways to diagnose whether you have a bad ignition coil or not. The methods also vary depending on which type of ignition coil you have in your car. Needless to say, doing these methods can be dangerous, so be sure to follow the necessary safety measures:

  • Do not touch or remove ignition cables, the distributor cap, or the spark plug connectors when the engine is running.
  • Switch off your ignition before connecting or disconnecting electronic control units, plug connections, and connection cables.
  • Make sure your working environment is dry and no water is present in the engine bay.
  • Remove any jewellery, including rings, watches, etc.
  • Cooldown your engine to avoid burn injuries and engage the parking brake.

Be sure to follow these safety measures as you will be working with a lot of electricity. Now, once you’re ready, here are the methods that you can do to check your ignition coil:

Swapping Method

This method is ideal if you only have one cylinder that isn’t firing, which means the problem is isolated to just one of the coil or cables. Here’s how to do it depending on your ignition coil type:

If you have a coil-on-plug system

First, diagnose which cylinder isn’t firing. Ideally, you will need an OBD reader to do this, as they can tell you specifically which cylinder is having issues. Once identified, simply swap the coil in that cylinder with another that is working fine. For example, if cylinder #4 isn’t firing, then swap it with the coil from cylinder #2. Afterward, clear the error codes and check again with the OBD to see which cylinder is now misfiring.

If cylinder #2 is now misfiring or not working, then that particular coil has gone bad. However, if cylinder #4 is still not working after the swap, then there’s a different issue. This could be a spark plug problem, a faulty fuel injector, or other mechanical issues within your engine.

If you have a can-type or distributor coil

As mentioned, a can-type or distributor coil is located remotely from the engine and connected to your spark plugs via HT cables. If you have one cylinder that’s misfiring or not working, try swapping the HT cable. For example, if cylinder #4 isn’t firing, then take out that cable on both ends. Then, swap it with the cable from a working cylinder. Keep in mind that each cylinder has a dedicated plug on the coil which has to be connected correctly. If you connect the cable to the wrong plug, then the engine will still misfire. Be sure to remember which plug each cylinder is connected to before you unplug them.

Once you’ve swapped the cables and made sure that each cylinder is connected to the correct plug, see if it still misfires. If it does, then the problem is likely to be the HT cable. They’re usually around $10 – $20 to replace.

Visual Inspection

You can do a simple visual inspection of ignition coils and the HT cables for signs of physical damage. Look carefully for signs of cracks, burns, or melting. If you have an older can-type coil, look for signs of oil leakage. Any of this physical damage can cause the ignition coil to go bad.

Winding Test or Ohm Resistance Reading

An ignition coil has two separate coils wrapped around each other, called the primary and secondary winding. Over time, these windings can short out or become open which causes it to go bad. While not the most accurate way to test your coils, you can still tell (to a degree) if your coil has gone bad. To do this method, you will need an Ohm multimeter such as the INNOVA 3320 Auto-Ranging Digital Multimeter. Here’s how to test it out:

  • Check your owner’s manual (or online) to see your car’s correct resistance reading for both primary and secondary windings. This varies between cars.
  • Connect the multimeter’s positive and negative cable to the terminals. Different coils will have different touch-points for this, depending on how the coil is designed. Be sure to check with your manual or seek online information to find out which pins to attach to.
  • Check the readings on the multimeter and compare them to your vehicle’s specifications.

A zero reading means that the coil has shortened internally. While a high reading above the specification means the coil is open. The test can be inaccurate because you can’t test the coil while it’s under load or while the car is running. However, this is still a good test to determine if there’s a problem with the coil.

Spark Test

The spark test is probably the most accurate way to determine if you have a faulty coil. Even with a faulty ignition coil, your vehicle can sometimes still run just fine. It’s usually when the vehicle is under load – such as when accelerating – that you start to notice the symptoms. The winding test can also be inaccurate as you can’t simulate the load or measure the coil during a car’s operation.

As a result, the surefire way to check is to see if your coils are still generating enough spark for your engine to operate. However, before you do a spark test, keep in mind that you will be testing the coils while your car is running. So, be sure to follow safety procedures religiously, as failure to do so can result in electric shock and painful injuries. No need to add expensive hospital bills on top of your vehicle’s repair costs.

To conduct this test, you will need a spark tester. The OEMTOOLS In-Line Tester should do the trick, or you can use the Lisle Coil on Spark Plug tester if your car has a coil-on-plug system. Here’s how to test them:

Can-type or distributor coil

  • Connect the “female” end to your spark plug and the other end into your HT cable.
  • Turn on the engine.
  • If the tester lights up, then that cable or coil still has enough spark.
  • Repeat the process on the other cylinders.

Here’s a video on how to do it:

Coil-on-plug system

Meanwhile, if you have a coil-on-plug system, you should follow these steps:

  • Remove the ignition coil you want to check and plug in the tester.
  • Connect the ground wire to the engine.
  • Start the engine and see if there’s a strong spark on the tester.
  • Repeat the process on the other coils.

You can watch the video below for a tutorial on how to do it:


After you conduct this test, you’ll find out which coils or cables have gone bad. The test is largely similar to the swapping test, but you don’t need to find out which cylinder is misfiring. Instead, you simply test each ignition coil or cable to see if they’re firing correctly and find out which ones have gone bad.

Keep in mind that different cars will have different ignition coils and removing them to test them may require different procedures. However, there should be plenty of tutorials online or in your owner’s manual to see how to test yours specifically.

Ignition Coil Replacement Cost

Ignition Coil Replacement Cost

Once you’ve found out which coils have gone bad, what you need to do now is replace them. The cost would obviously vary depending on the type of ignition coil used in the car. For example, if you own an older car such as the classic Mini Cooper which uses a can-type ignition coil, then replacing it will cost you around $30 – $40 for the part itself.

However, if your car is running on coil-on-plug systems, then it’s usually pricier. The price of a set of ignition coil ranges between $100-$300, depending on your type of car. Meanwhile, the labor cost is usually around $100 – $150, which could bring your total to as high as $450 to replace an entire ignition coil set.

Can you fix it yourself?

Of course, ignition coils are not too complicated to replace and don’t require specialized tools. As long as you have the correct tools at home, you could do it by yourself. If you have a can-type coil, it’s as simple as undoing the cables and a few screws and taking out the old coil. Then simply fit a new one onto the mount and reconnecting the HT cables.

A coil-on-plug system on modern cars can be a bit more complicated to replace by yourself, but not undoable. You simply remove the engine cover (if there is any) and then disconnect the connectors. There will sometimes be a bolt that holds the coil in place which you will need to undo, but otherwise, you can simply pop the coil out. Afterward, simply put in the new coils and reconnect the cables.

If you’re interested in replacing it yourself, watch the video below on how to do it. You might as well check and replace your spark plug if necessary while you’re at it:

There are plenty of tutorials online on how to replace your ignition coil, or you can check with your car’s manual as well. Be sure to research how to replace the ignition coil for your specific car so you understand it clearly. Afterward, make sure you have the right tools to do it and you shouldn’t run into any trouble while replacing your car’s ignition coil.

How long do ignition coils last?

Ignition coils in most modern cars will last you up to 100,000 miles before they need to be replaced. If your ignition coil fails prematurely, this can be caused by extreme temperature or moisture. This affects mostly single coil systems such as the can-type which are vulnerable to overheating issues. For other systems, unless there’s a manufacturer design flaw or extreme weather, then it’s likely because there’s another problem with your engine. For example, a blown head gasket can increase your engine’s operating temperature, affecting the ignition coil.

Another common reason is bad HT cables. When an HT cable goes bad, it can cause high resistance which generates a higher amount of voltage on your secondary winding. As a result, it can wear out your ignition coil prematurely.


A faulty ignition coil affects your engine’s operation, therefore it’s important to replace it should yours have gone bad. If you don’t, your car might completely stop working at inconvenient times. It can also lead to further damage to your catalytic converter and exhaust system, leading to expensive repairs that can cost thousands of dollars to repair.

Faulty ignition coils are not too expensive to repair, even with modern cars. They normally cost no more than $450 to replace an entire set. However, luxury and performance cars may cost more than that. Be sure to check with a few repair shops to make sure you get the best service and estimates.

You can save some money by doing the replacement job yourself. This can be complicated depending on your car make and model, but it’s doable. Be sure to follow the safety procedures before you work on your ignition coil, as you will be working with a high amount of electricity that can cause serious injuries.

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