Ignition coils are an important part of ignition systems on modern cars. Since the 90’s, most engines have veered away from distributor ignition systems and have instead relied on electronically-controlled ignition. These modern electronic systems use ignition coils or coil packs to control ignition.
While this is the most common form of ignition system today, some cars still use distributor-less electronic systems that use one master ignition coil that distributor power to all of the spark plugs via spark wires.
There are a few different types of ignition coil setups:
Coil-on-Plug (COP): with coil-on-plug systems, the ignition coil sits directly on top of the spark plug in the cylinder. Each cylinder and spark plug has its own individual coil.
Coil-near-Plug (CNP): there isn’t always enough room to put the ignition coils directly on the spark plug. When this is the case the ignition coils are separated from the plug and connected via spark plug wires.
Coil-per-Cylinder (CPC): coil-per-cylinder systems can be both COP or CNP but are specific to systems that use one ignition coil per cylinder. Some modern ignition systems will use one coil to power two cylinders as an alternative to CPC systems.
Coil-on-Plug systems are the best option where available as they eliminate the need for spark plug wires which is simply one additional potential failure point for the ignition system.
Ignition coils are commonly referred to as coil packs. While coil packs are technically only correct terminology for ignition coils that are physically grouped together, we will use the terms intermittently in this article.
What Are Ignition Coils?
Engines require combustion to start and continue running. The three main components for combustion are: air, fuel, and spark. While the spark plugs provide the actual spark, they require electrical current to create it.
For a spark plug to fire it needs thousands of volts of electricity. Batteries on cars are usually only 12-volt batteries, far short of the voltage needed for a spark plug to fire.
Ignition coils are essentially electrical transformers. The ignition coils take in the voltage from the battery and transforms the voltage from 12-volts to the tens of thousands of volts required for the spark plugs to function properly.
How Do Ignition Coils Work?
The inside of a spark plug contains a coil of wire that is used as a conductor. The wire coil has a circuit switch that that opens and closes to circulate electrical currents through the coil.
When the switch is open, current flows through the windings of the coil. This creates a magnetic field around the coil which amplifies the voltage of the electrical current. The switch is then closed which collapses the magnetic field back into the coil where it can be transferred to the spark plug.
Most ignition coils nowadays have a two-coil wire system. The first coil creates a magnetic field which can 10x-20x the voltage of the initial 12-volts. The second coil wire is more compact and therefore creates more voltage through the magnetic field, resulting in a 100x increase in voltage, allowing voltage levels to exceed 20,000 volts.
If you want to get into the nitty gritty, check out Denso’s technical writeup on how ignition coils work.
Bad Ignition Coil Symptoms
- Cylinder misfires
- Hard starts
- Rough idling
- Hesitation or stutter under acceleration
- Exhaust backfires
- Check engine light (codes for misfires)
- Worse fuel economy
The most noticeable symptom of a bad ignition coil is misfires, especially under acceleration. When an ignition coil is failing, it does not provide the spark plug with enough voltage to spark. This causes gasoline to buildup in the cylinder which can then detonate from heat at the improper time, resulting in a misfire.
Misfires will feel like a jerk or hesitation from the engine. You will usually start noticing misfires from time to time and eventually they will become very frequent once the coil has further deteriorated. If just one ignition coil goes bad you likely won’t have any starting issues, however, these can occur if you have multiple go bad at the same time.
Most cars will throw P0300-P0312 engine codes for misfires. The specific code will point to which cylinder is misfiring. Some cars will throw codes specific to the ignition coils, which are usually P0350-P0362.
The symptoms for bad ignition coils are pretty much exactly the same as the ones for bad spark plugs. Make sure you pull your spark plugs and check them before buying and installing a new set of ignition coils. Because the symptoms are the same and spark plugs are generally cheap, most people replace them at the same time they replace the ignition coils.
How to Test Ignition Coils
The first and foremost check should be inspecting the coils themselves. If they have cracks, burn marks, are melted, or coated in oil they are probably bad. If the coils look okay on visual inspection, try these methods for testing ignition coils:
1) Swapping Coils
The simplest method for determining whether the issues you are experiencing are spark plug or ignition coil related is to swap the ignition coils.
For example, if you are getting an engine code for a misfire in cylinder 2, swap the cylinder 2 ignition coil with the cylinder 1 ignition coil. Clear the engine codes and drive the car until you get another misfire and check the engine codes again. If the misfire code is now saying cylinder 1, then the ignition coil is bad. If you use this method and the engine code still says cylinder 2 is bad then this is a sign that your spark plug is bad (or there’s another issue at hand).
For cars that use spark plug wires you can follow the same methodology but with swapping the wires instead. Just make sure you don’t swap the ignition coil with it otherwise you won’t be sure if it is the wire or the coil that is bad.
2) Using a Multimeter
Testing the coil wires of the ignition coil for Ohms resistance is the second easiest option if swapping coils isn’t a choice. Buy a multimeter and connect the positive and negative leads for the proper coil wire terminals. You will need to test both the primary and secondary windings for their ohm readings.
Each ignition coil will have different touch points depending on how it was manufactured. Check your owners manual or Haynes repair book for the proper touch points on the coil as well as for the proper Ohm ranges for each coil winding.
Because ignition coils operate differently under load vs. without load, it is possible for this method to give false answers.
3) Other Testing Options
The above two options are you best bet. We prefer methodology #1, but this can be a bit of a hassle if you have spark plug wires too and have to do multiple swaps to diagnose the problem.
There are ignition coil testers and inline spark testers that are a bit more advanced in use, as well as more expensive newer coil-on-plug testers that allow you to test coils without pulling them off the vehicle.
Ignition Coil Replacement Cost
This isn’t going to be true for every single car, but for most modern cars any average joe with the proper tools can swap the spark plugs and ignition coils.
When you replace your ignition coils, you have two options: replacing only the ones that are bad, or replacing all of them. Most people like to swap them all out at the same time, but this becomes more costly especially for people with V8 or larger engines.
Also, if you plan on having them replaced at a repair shop, doing them all at the same time will save on labor costs as the majority of the time it takes to replace them is getting to them in the first place.
Single Ignition Coil Replacement Costs
- Parts: $20-$40
- Labor: $80-$150 or approximately 1 hour of labor costs
Ignition Coil Set Replacement Costs
- Parts: $100-$400 (cost increases as engine gets bigger ie. inline-4, V6, V8)
- Labor: $120-$250 or approximately 1.5 hours of labor costs
Ignition Coils vs. Spark Plug Wires
Cars that don’t use coil-on-plug systems will use spark plug wires. Ignition coils create the voltage required for the spark plugs and the plug wires transfer that current from the coil to the spark plug.
Because the wires are responsible for carrying the voltage, bad wires will also cause the same signs and symptoms as bad ignition coils. These are also commonly replaced together at the same time, but use our testing methods above to determine if it is the coils or wires causing the issues if you just want to swap out the bad culprit.
Cars that’d don’t have spark plug wires will have a coil-on-plug setup where the ignition coils attaches directly to the spark plug head, removing the need for transport wires and reducing a potential failure point.