Ignition Coil Failure Symptoms

Ignition coils are one of the most important components for an engine to create spark for combustion. They are also, however, one of the most frequent failure items. Rough idling, a loss of performance, misfires, and issues starting your car are some common signs that could point to ignition coil failure.

In this article we’re going to discuss the basics of ignition coils, the signs and symptoms that occur when they go bad, and provide some diagnostic steps to help you test whether or not your ignition coils are bad.

Understanding 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.

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 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) Swap Cylinders

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.

Replacement Costs

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 Replacement

• Parts: $20-$40
• Labor: $80-$150 or approximately 1 hour of labor costs

Full Set Replacement

• 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