If you drive a relatively new vehicle, there's a good chance that your car uses a coil-over-plug ignition design. Unlike older engines, these designs require spark plugs wires to carry current to each plug. Instead, each cylinder gets its own ignition coil that sits directly on the spark plug. These components don't require routine maintenance, but they are a common failure point on many engines.

A faulty ignition coil can cause numerous frustrating symptoms, so it's helpful to understand what you should expect when yours are going bad. Fortunately, diagnosing a bad ignition coil is usually relatively straightforward, and it's an easy problem to fix on many cars. (And when in doubt, it is always safest to visit an auto repair shop.)

Recognizing Ignition Coil Symptoms

Ignition coils provide the voltage necessary for your spark plugs to ignite the fuel/air mixture in your cylinders. As a result, a faulty ignition coil will result in an inconsistent spark, creating similar symptoms to a failing spark plug. Common problems include misfires, noticeable reductions in fuel economy, poor idling, and rough acceleration.

Unfortunately, your car typically can't tell if an ignition coil is faulty. You may receive a check engine light, but pulling the code will likely turn up some generic errors related to misfiring. The good news is that these codes can point you in the right direction. If your misfire codes point to a single cylinder, that's a good indication that you have a bad plug or a faulty coil.

Differentiating Between Bad Plugs and Coils

If your computer can't tell you which component is at fault, how can you know for sure without throwing parts at the problem? Experienced mechanics will generally approach a suspected coil failure by swapping coils between cylinders. For example, if your computer reports a misfire on cylinder 4, it's typically an easy job to swap the coil on cylinder 4 with a coil that isn't reporting a misfire.

This straightforward diagnostic step is enough to help isolate the problem in most cases. If the misfire follows the coil to the "good" cylinder, that's a sure sign that the coil is faulty. The technician may also perform some bench tests on the coil to confirm the problem. On the other hand, if the misfire remains on the original cylinder, the problem may be with the spark plug or another component.

Fixing Faulty Coil Misfires

The good news is that repairing a misfire caused by a faulty coil is as easy as swapping the old coil out for a new one. Since ignition coils aren't maintenance parts, it's unnecessary to replace your other coils just because one fails. While ignition coils may sometimes fail in pairs or sets, you can continue using your other coils as long as they aren't currently causing any problems.

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