Q: On occasion, my car will sputter, an issue it experienced years ago before a shop fixed it by replacing its ignition coils.

That fix was costly and, in terms of time, very inconvenient, which has me wondering if this time I could do the repair myself. Is there a way to know which coil is bad? They look pretty simple to replace and my auto parts store has them.

A: Hmm. I admire your spunk, but let’s tiptoe into this one carefully.

Has the check-engine light flashed or stayed on? If so, there will be a diagnostic trouble code stored in the car’s system that might tell you what’s going on.

Some information the trouble code might reveal: A specific cylinder has misfired more times than what’s considered allowable; a misfire has occurred, but the system hasn’t been able to detect which cylinder malfunctioned; something else entirely happened.

If the check-engine light isn’t illuminated, it’s still a good idea to employ a generic diagnostic scan tool to look at the number of misfires, if any, the system has recorded. If you do have a misfiring cylinder, even one misfiring briefly enough to not trigger a trouble code, it’ll still show up in the scan tool’s reading.

With all of that in mind, let’s say a trouble code indicates the second cylinder is misfiring or shows a significantly higher misfire count than the others. In that case, there’s a decent chance the ignition coil for that cylinder may be faulty. A somewhat common cause for that is spark leakage that occurs due to an insulation problem in the coil’s plastic housing.

Keep in mind, however, there could be reasons other than a faulty coil that a cylinder is misfiring.

Next, assuming you have continued access to the scan tool, try clearing the trouble codes and swapping the suspect coil with a neighboring cylinder. After driving the vehicle far enough to encounter the sputtering, recheck for trouble codes and misfire counts. If the trouble code or counts now point to the cylinder onto which you swapped the suspect coil, you have a slam-dunk diagnosis. If the code, however, points to the original cylinder, and renewing the spark plug doesn’t fix it, professional diagnosis will likely be needed.

An easy way to test a coil-on-plug coil is to remove the coil from the engine, reconnect it to the wiring harness and plug it into a spark tester.

The tester’s alligator clip must be connected to engine metal, and you’ll then want to look for spark jumping the tool’s air gap as the engine is cranked or briefly started. The tool’s very large gap really gives the coil a workout, and only a good one will pass muster. To verify spark leakage isn’t an issue, wrap the coil in aluminum foil as if dressing it in a snug sweater and make sure the foil is touching engine metal. If spark still jumps across the tool’s air gap, the coil is leak-free. This test catches a lot of bad coils.