Diagnosing a Bad Ignition Coil

Ignition coils typically have a long life of operation — over 100,000 miles. However, they can malfunction eventually, causing that dreaded misfire. This issue will probably start out slowly while being hardly noticeable but will become more problematic over time. When a misfire occurs, even for a few seconds, your engine computer management will likely detect the issue and generate a trouble code. At this point, the check engine light comes on.

With a diagnostic scanner at a garage or auto parts store, you can then see which cylinder is misfiring and proceed from there. Another sign of a misfire can be a rough engine idle. In extreme cases, the vehicle may feel like it's surging at a high RPM or like it may stall at low speeds.

Other signs that you have a bad ignition coil are worsening gas mileage, sluggish acceleration, and a backfiring engine, meaning you hear a loud bang as unburned fuel escapes from the exhaust system.

Apart from the coils, another likely culprit for misfiring is the neighboring spark plug, faulty spark plug wires, or a larger electrical issue. When you take your car to a professional shop, they have an array of voltage testing equipment to see where the malfunction is happening along the circuit. They may also look for other things like clogged fuel injectors.

Ignition Coil Replacement Cost

Once you've identified a faulty ignition coil, replacing them usually costs about $250 per part. Then there's labor on top of that, which can easily add an extra $50 or more per coil. If you're handy, this is a job you may want to attempt yourself since the ignition coils are usually located in an accessible spot on top of the engine and can be removed with a basic socket wrench. Just detach the 12V battery first to be safe.

At the same time, you may want to consider replacing all of your car's spark plugs. The plug in the misfiring cylinder could've become corroded or caked with residue, and it's probably due for a replacement anyway. Spark plugs will generally be harder to reach than ignition coils and require specialized sockets to unbolt from the combustion chamber. If all that sounds too complicated, your best bet will be to seek a professional mechanic.

With multiple ignition coils and spark plug replacement jobs, a shop can easily rack up thousands of dollars of parts and labor on your bill. It's also likely to put your vehicle out of commission for at least a few hours. If your car is still subject to a manufacturer's powertrain warranty, then that can save you a bunch of money while getting maintenance like ignition coil replacement done at an approved garage.