How do I know which ignition coil is bad?

The easiest test: swap the suspected coil with one from an adjacent cylinder, then clear the codes and drive. If the misfire code moves to the new cylinder (the one you moved the coil to), the coil is bad. Get a free OBD2 scan first — a P0301 through P0308 code tells you exactly which cylinder is misfiring.

Should I replace all ignition coils at once?

If the car is over 80,000 miles and one coil failed, the others are the same age. Many mechanics recommend replacing all coils while the job is open — coil packs are cheap ($20–$40 each) and the labor is the same whether you replace one or all.

Can a bad ignition coil damage the catalytic converter?

Yes. A coil that fails causes a cylinder misfire, sending raw unburned fuel into the exhaust. That fuel ignites in the hot catalytic converter, overheating and destroying the catalyst. Fix misfires immediately — a $30 coil versus a $2,000 catalytic converter.

Ignition Coil

easy DIY~0.5h job

What it does

Your ignition coil is an electrical transformer that takes the 12 volts from your car's battery and boosts it to 30,000–45,000 volts — enough electricity to jump the gap at the tip of the spark plug and create a spark. Modern cars have one coil per cylinder (called coil-on-plug), so if one coil fails, that cylinder stops firing. You lose 1 of your 4 (or 6 or 8) cylinders, and the engine runs rough — like a person trying to sprint on one leg.

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DIY Cost

~$30

parts only

Shop Cost

~$200

parts + labor

Easy DIY~0.5h job

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DIY tutorial

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Frequently asked questions

car maintenance — complete guide check engine light — complete guide