Ignition Coil
Also known as: coil pack (older multi-coil units) · COP (coil-on-plug) · pencil coil
Quick answer
An ignition coil is a compact transformer that turns the battery's 12 volts into the 25,000–45,000 volts a spark plug needs to ignite the mixture. Modern engines give each cylinder its own coil sitting directly on the plug — which is why a single bad coil shows up as a single-cylinder misfire.
Spark needs voltage, and lots of it: jumping a gap inside a pressurized cylinder takes tens of thousands of volts. The coil does this with two windings around an iron core — the computer charges the primary winding, then cuts the current at the exact firing moment, and the collapsing magnetic field induces a massive voltage in the secondary winding. That pulse travels down to the spark plug. This happens for every cylinder, every cycle — millions of times per month of driving.
Older engines used one coil and a distributor to route the spark; modern ones use coil-on-plug: one coil per cylinder, sitting in the plug well. Better spark, no moving parts to wear, and beautifully simple diagnosis — when one coil dies, exactly one cylinder misfires, and swapping that coil to another cylinder makes the misfire move with it.
Coils die from heat cycles, vibration, and — very commonly — oil. A leaking valve cover gasket fills the plug well, soaks the coil boot, and shorts the secondary side. That's why a coil replacement should always include a look down the well: if it's wet, the gasket is part of the repair, or the new coil inherits the same death.
Signs it’s failing
- ⚠ Single-cylinder misfire (P0301–P0312) — the signature presentation
- ⚠ Coil circuit codes (P0351–P0362) when the failure is electrical
- ⚠ Misfire that appears when the engine is fully hot and disappears cold — classic heat-related coil breakdown
- ⚠ Rough idle, shaking under acceleration, flashing check engine light under load
- ⚠ Fuel smell and falling economy as one cylinder's fuel goes unburned
- ⚠ Visible cracks or carbon tracking on the coil body or boot
Trouble codes this part can trigger
Frequently asked questions
- How do I confirm a coil is bad?
- The free swap test: move the suspect coil to another cylinder, clear the codes, drive. If the misfire follows the coil (P0301 becomes P0303), the coil is condemned by its own testimony. It's the single most reliable misfire diagnostic that exists.
- Should I replace all the coils when one fails?
- Unlike spark plugs, coils aren't scheduled-maintenance items — they fail individually. Replace the bad one; consider the full set only on high-mileage engines where access is painful (buried rear banks) and you never want to open it again.
- Do coils and spark plugs fail together?
- They stress each other: a worn, wide-gapped plug makes its coil work harder and die younger. That's why a coil job should include checking (usually replacing) the plug under it, especially past the plug's interval.
- Cheap coils or OEM?
- Coils are precision high-voltage parts, and the bargain bin has a real comeback rate. OEM, Denso, Hitachi, Delphi, or other established brands cost somewhat more and tend to actually last. The misfire returning in six months erases any savings.