How do I know if my ignition coil is bad?

A cylinder-specific misfire code (P0301–P0308) that follows the coil when swapped to another cylinder confirms coil failure.

Should I replace all ignition coils at once?

Not required, but recommended if the vehicle has high mileage (80K+) and one coil has already failed — the rest are likely nearing end of life.

Can a bad ignition coil damage other components?

Yes — a weak coil causes incomplete combustion, sending raw fuel into the catalytic converter and potentially destroying it over time.

Buyer Guide

Best Ignition Coils

Ignition coils transform your battery's 12V into the 20,000–40,000V needed to fire a spark plug. A failing coil causes cylinder-specific misfires (P0301–P0308), rough idle, and power loss. Replacement is straightforward on most coil-on-plug engines.

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What to look for

OEM vs. aftermarket — OEM coils are the safest bet; quality aftermarket (Denso, Delphi, Spectra) perform comparably at lower cost
Coil-on-plug (COP) vs. coil pack — most modern engines use COP; older engines may use a shared coil pack
Boot condition — some replacement coils include a new boot/spring; others require reusing the old one
Voltage output — should match OEM spec for reliable spark under all load conditions

Our picks

Editor's Choice

Denso Ignition Coil

Best Overall

$20–$45 each

4.7/5

OEM supplier for Toyota, Lexus, Subaru — direct-fit with factory-matched output

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Delphi GN10114 Ignition Coil

Best Value

$15–$35 each

4.4/5

Reliable aftermarket option with OEM-equivalent performance and wide coverage

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Motorcraft DG-508 Ignition Coil

Best for Ford

$25–$50 each

4.6/5

OEM replacement for Ford vehicles — essential for F-150, Explorer, and Mustang models

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Buying tips

If one coil has failed, the others are likely aging too — consider replacing all coils and spark plugs together for even performance
Use the coil-swap diagnostic: move the suspect coil to a different cylinder and see if the misfire code follows it
Apply dielectric grease to the coil boot to prevent moisture intrusion and make future removal easier
Check the coil connector for corrosion or heat damage before installing a new coil — a bad connector will destroy a new coil

Related diagnostic codes

P0301 P0302 P0303 P0304 P0300

Related guides

Best Spark Plugs Best Obd2 Scanners

Related OBD-II code guides

See the codes this part commonly triggers — and what to inspect before replacing anything.

Recommended tools & parts

These recommendations appear because this issue commonly requires inspection in this system area. ratchit does not recommend replacing parts until the likely cause is confirmed.