How to Test If an Ignition Coil Is Bad

Ignition coils are an important part of the ignition system. Without them, the spark plugs would not have enough voltage for combustion. When an ignition coil fails, a few signs and symptoms can appear, and depending on the type of ignition coil, there are some DIY methods you can try to tell which ignition coil is bad.

Because coil-on-plug ignition coils are so common and popular on cars today, this article will cover how to diagnose coil-on-plug ignition coils that manage one spark plug or, as in waste spark systems, two spark plugs.

If the Check Engine Light Is on, Identify the Misfiring Cylinder with a Scanner

If the check engine light is on, a scan tool can read which specific cylinder is misfiring. This is identified by a single digit number at the end of code P0300 (ex. P0302) that represents the cylinder that is misfiring (in this case, cylinder 2). To find the corresponding cylinder on the engine, a general rule of thumb is that cylinder 1 is usually the cylinder closest to the front of the engine, or consult a repair manual or other reputable source specific to your vehicle’s engine.

Test the Ignition Coil with One of the Following Methods

1. Swap the Ignition Coil with One from a Different Cylinder

Coil-on-plug ignition coils can be tested by rearranging the ignition coils. Once the misfiring cylinder has been identified, swap its ignition coil with a different one from another cylinder, reinstall them completely, and see which cylinder misfires. If the ignition coil is faulty, the misfire code will follow it.

If the misfiring cylinder keeps misfiring after ignition coils have been swapped, the problem probably lies with other ignition parts like the spark plug. To confirm the misfire isn’t related to the ignition coil, check the electrical connector (on wasted spark systems also the spark plug wire) on the ignition coil for corrosion or melting. A defective electrical connection can cause an ignition coil to fail.

Check out this video to see how to diagnose a bad ignition coil by swapping them


2. Mist the Ignition Coil with Water

With waste spark ignition systems, misting the ignition coil and spark plug wire with a spray bottle of water will reveal if there’s cracking on the ignition coil or a break in the current. It might help to bring the vehicle to a darkened area to see the spark, but one way to test these ignition coils is to run the engine and lightly spray the ignition coils with water. Stand a safe distance away, do not touch any electrical components, and observe the coils and wires with your eyes and ears for a spark, which will indicate they are the cause for the misfire.

Check out this video below to see how a bad ignition coil interacts with spray water

Need New Ignition Coils?

If the ignition coils are defective, we recommend replacing them in pairs. At 1A Auto, you can save on quality ignition coil kits and direct fit replacements.


Ignition Coils



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