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Misfires can be the easiest things in the world to cure, or the thing that drives you to drink. Luckily, OBD-II cars are straightforward to diagnose, and diagnosing a dead coil is gravy work that can be done in a little under an hour. You'll need an OBD-II scanner, a sharpie, and a spray bottle with water.

3108NIf you have a dead miss on a car with individual coils, check with your OBD scanner which cylinder is misfiring. If you have multiple faults for misfires, but an active flashing check engine light, clear the faults and see what comes back quickly. You can also monitor the individual cylinders misfiring, if you have a good OBD scanner.

Pull up the coil on the cylinder that's misfiring, as well as the two next closest. Don't get anything mixed up; organization is key. Write on the coils which cylinder they came from, using the special tool (sharpie), and switch the coil with the one from the cylinder next to it. Switch the plugs from the cylinder in question with the other cylinder – NOT the one you swapped coils with.

Replace what you need to

The idea is to move the questionable plug down one, and the coil up one. This ensures that all components are shuffled around in a controlled way, in order to isolate exactly which component is dead. Start the engine up and see where your misfire moved to. Whatever is currently on that cylinder (plug or coil) is what is compromised. Replace whatever it is that you need to, and clear the faults.

Now, on cars with a single coil and wires, diagnosing a coil is a little different. If you have random misfires, check all your plugs first, and check your wires. If your engine has a distributor, check the cap and rotor too. If any of them look bad, step one is replacement. These are wear and tear items with a service interval, the coil is not. If your wires, plugs, cap, and rotor are new, or newish, and you still have random misfires, then it's likely a coil.


12137594937KT-1 BMW Ignition Service Kit with Coil Packs


To check this style coil, you'll need another special tool – a spray bottle with water. Start the car, let it run for a while, and spray the coil down with water. Set your sprayer on stream, not mist, in order to focus your diagnostic stream. It might take a second for the water to soak in, so don't flood it immediately. If your coil is bad, your engine will run rough, and possibly stall out. If your coil is near your distributor, tread lightly. Water in your distributor will set your diagnosis back until you can dry it out.

Water will penetrate through the cracks

The reason these coils fail, in my experience, is because they overheat and crack. Once that happens, they short out against whatever they're mounted to, and fail to deliver the spark to the plug. Using water can help to diagnose this type of coil, because it will penetrate through the cracks, if there are any, and cause the coil to fail.

Now, as with any repair, proper diagnosis is only step one. Use quality parts when it's time to replace, and you won't have to go back in for a while.

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Written by :
Chris Stovall

Chris is a journeyman mechanic from Berkeley, California, specializing in late model Volkswagens and Audis. A glutton for punishment, his spare time is spent rebuilding every component of his ’83 Rabbit GTI.

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