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Ignition coil could be your culprit

Can I fix a defective ignition coil by myself?


Q: About two years back my engine began to shudder and lose power at times. My trusted mechanic quickly determined the cause was a defective ignition coil and replaced it. All has been fine until this past week when the same issue reared its head again. When doing the original work, my mechanic didn't replace all of the coils as all except the defective one were OK at the time. I'm wondering if another one of them has now gone bad. Since my mechanic has retired I'm wondering if I could fix this myself. The engine light is also on.

—Patty Warner

A: Patty, it sounds like you've done a great job identifying and comparing the two engine performance symptoms and may be on the right track with your ideas.

Your engine is apparently equipped with a coil-on-plug ignition system, which is pretty much the norm for modern engines. Mounting individual ignition coils directly atop the spark plugs eliminates parts and potential problems found on older distributor type systems that delivered a spark from a single coil via the distributor cap, rotor, and plug wires.

COP ignition coils come in basically two versions, those containing an integrated ignition control module (three or four connecting wires) and those that don't (two wires). The ignition control module, wherever it's located, performs the critical task of switching the coil(s) on and off at precise times based on signals from the powertrain control module. COP systems are very reliable, but the ignition coils can sometimes leak spark via defects/damage in the plastic housing or spark plug boot, to nearby engine metal.

Your first step in fixing this is to buy or borrow a basic OBD-II scan tool such as the Autel MaxiScan MS309 at for $24. It's amazing these tools have become so inexpensive! The scan tool will display the diagnostic trouble code stored and a freeze frame showing engine/vehicle conditions at the time the check engine light illuminated. You will likely find a code such as a P0302. This example indicates a misfire in cylinder No. 2. Since your check engine light appears to be on steady rather than flashing, the misfire is likely occasional rather than severe. Scan tools often list trouble code definitions or you can simply Google them. The website does a great job explaining definitions and possible causes.

I normally frown on throwing parts at a car to fix problems, but your engine contains a built-in parts department. Swapping the ignition coil of the indicated cylinder with the one next door could not be easier if they're readily accessible. Removal usually consists of removing an attaching nut and the wiring connector and lifting the coil from the spark plug, installation is the opposite. I'd then use the scan tool to clear the trouble code and drive the car under the conditions indicated in the freeze frame until the check engine light again illuminates. After parking, check to see if the current code is perhaps now a P0303. If so, this means the misfire followed the coil you swapped to cylinder No. 3. In this case renewing the now-No. 3 ignition coil will fix the misfire. If the current code still indicates cylinder No. 2, the fault lies elsewhere and will likely require professional expertise to fix.

I should add that when evaluating a typical diagnostic trouble code in situations other than this, resist the temptation to replace the component mentioned in the code description unless further testing confirms it is defective.



Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood(at); he cannot make personal replies.


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