Ignition Coil
What it is
An ignition coil transforms the car's 12-volt supply into the tens of thousands of volts a spark plug needs. Modern engines use a coil-on-plug (COP) setup — one coil sitting directly on each plug.
What it does
It delivers the high-voltage pulse that fires the spark plug. When a coil weakens or fails, that cylinder misfires — usually showing a cylinder-specific code (e.g. P0301 for cylinder 1). Heat and age are the usual killers.
Symptoms of failure
- Cylinder-specific misfire code (P0301–P0304)
- Rough idle and engine shaking
- Hesitation and power loss under load
- Flashing check engine light
- Hard starting in damp weather
Common fault codes
Which vehicles need it
Coil-on-plug engines from virtually every brand. Some (e.g. certain VW/Audi, Mini, Ford) are known for premature coil failure.
Replacement cost
| DIY (part only) | $20–$150 |
|---|---|
| At a shop (parts + labor) | $110–$350 |
| Replacement interval | No fixed interval — replace on failure. Replace the spark plug at the same time. |
| DIY difficulty | Easy (DIY) — usually one bolt and one connector per coil |
| Recommended brands | Denso, Bosch, NGK, Delphi |
Where to buy the part
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Frequently asked questions
How do I know if it is the coil or the spark plug?
Swap the suspected coil to a different cylinder. If the misfire follows the coil, the coil is bad; if it stays on the same cylinder, suspect the plug, injector or compression. Plugs are cheap, so many people replace both.
Can I replace just one ignition coil?
Yes, you can replace a single failed coil. But if the others are the same age and high-mileage, replacing them as a set can save future breakdowns.
Will a bad ignition coil damage the catalytic converter?
It can. A misfiring coil dumps unburned fuel into the exhaust, which overheats and can destroy the catalytic converter — fix a misfire promptly, especially if the light is flashing.