Code or symptom/
module

Ignition Coil

Steps battery voltage up to the tens of thousands of volts needed to jump the spark plug gap. A misfire code names a cylinder, not a part — the coil is one of at least four things that can cause it.

Typical cost
$100–$1,200
Wear item
No
Service life
Not a scheduled maintenance item, but a component with a finite life that shortens under heat, oil contamination, and worn spark plugs. Replacing all coils preemptively because one failed is a judgment call, not a rule — it is defensible when access is difficult and the coils are original and high-mileage, and wasteful when the coil is easy to reach.
What it does

A spark plug needs far more voltage to fire than the battery provides, and the ignition coil is what creates it. On most modern engines each cylinder has its own coil sitting directly on top of the plug. When one fails, that cylinder stops contributing: the engine shakes, loses power, and the check engine light comes on — often flashing. A flashing light means raw fuel is going into the exhaust and the catalytic converter is being cooked in real time. Stop driving and get it towed rather than nursing it home, because a cheap coil failure turns into an expensive converter failure quickly.

The coil is a transformer with two windings around a shared core. The engine computer or ignition module grounds the primary winding, which is a small number of heavy turns, and current builds a magnetic field over a few milliseconds. At the moment of ignition it opens that circuit, the field collapses, and the collapse induces a proportionally enormous voltage in the secondary winding, which has many thousands of fine turns. That secondary voltage climbs until it is high enough to ionize the gap at the spark plug and jump it, then sustains a lower-voltage arc for the burn duration. Two things follow. The voltage the coil actually produces is demand-driven, not fixed: a worn plug with a wide gap or high cylinder pressure under load forces the coil to work harder, which is why coils often fail under load first. And the secondary winding is insulated by potting compound and a plastic boot — heat cycling and oil intrusion destroy that insulation, letting the spark find a shorter path to ground than through the plug.

How it fails

Insulation breakdown is the classic. The coil works fine at idle where demand is low, then breaks down under load or under heat when required voltage rises, so the misfire appears on acceleration or after the engine is fully warm and disappears when cold. Cracked coil boots let spark track to the valve cover instead of firing the plug, and you can often see the carbon trail on the boot. Oil in the spark plug wells, usually from a leaking valve cover gasket, saturates the boots and kills coils repeatedly — replacing the coil without fixing the leak buys you a few months. Open or shorted windings produce a hard, permanent misfire and often a coil circuit code. Connector and harness faults mimic coil failure exactly, and heat-damaged connectors near the exhaust side are common. Worn spark plugs kill coils indirectly: an excessive gap raises the required voltage, and the coil eventually fails trying to meet it. That is why coils and plugs get replaced together in practice.

Symptoms when it is failing
  • Rough running with a distinct shake or shudder, often at idle
  • Flashing check engine light — a converter-damaging severe misfire, stop driving
  • Misfire that appears only under acceleration, under load, or when hot
  • Hesitation and clear power loss
  • Hard starting, especially in damp or cold conditions
  • Raw fuel smell from the exhaust
  • Fuel economy drop with a stored cylinder-specific misfire code

These symptoms overlap with several other faults. They are a reason to test this component, not evidence that it has failed.

How to test it before replacing it

Read freeze frame data before clearing anything, because the conditions at the time of the misfire — rpm, load, coolant temperature — tell you whether the fault is heat-dependent or load-dependent. Then read misfire counters per cylinder in live data and confirm which cylinder is actually affected, since a single P0300 with no cylinder identified points more toward fuel, vacuum, or timing than toward one coil. The definitive field test is the swap: with the engine off and cool enough to handle, move the suspect coil to a known-good cylinder, clear codes, and run the engine until the misfire returns. How you run it depends on how severe the misfire is. If the MIL is flashing, do not road-test it — that is the exact condition that destroys the catalytic converter, and driving miles to reproduce a fault you have already confirmed is self-defeating. Run it in the bay or on a lift under controlled conditions, only long enough for the misfire counters to show which cylinder the fault moved to, and shut it down as soon as they do. A load-dependent misfire that will not appear at idle can be provoked on a lift or a dynamometer under supervision rather than in traffic. If the misfire follows the coil, the coil is bad. If it stays with the original cylinder, the coil is innocent and the problem is the plug, the injector, or the cylinder itself. Swap the spark plug the same way to divide the ignition side further. Confirm the cylinder can actually make power before blaming ignition at all: a compression or relative compression test rules out a burnt valve or broken ring, which produce identical misfire codes and cannot be fixed with parts from the front counter. For circuit codes, check power and the switching signal at the coil connector, and inspect for corrosion and heat damage. Secondary ignition carries tens of thousands of volts — keep hands and test leads out of the secondary path with the engine running, and never disconnect a coil connector on a running engine. Coil primary and secondary resistance specifications vary by manufacturer and many modern coil-on-plug units contain integrated electronics that make a simple ohm reading meaningless, so do not condemn or clear a coil on resistance alone unless you have the specification for that exact part. A lab scope on the primary or secondary waveform is the real diagnostic tool here and shows a breaking-down coil that passes every static test. Finally, pull the plug and look at the well for oil.

Specification values differ between manufacturers and model years. Where a figure is not genuinely standard across OBD-II vehicles, check it against service information for your specific vehicle rather than a generic number.

Where it sits

On most modern engines, mounted directly on top of each spark plug under a cover, one per cylinder. Older systems use a coil pack firing two cylinders at a time through plug wires, or a single coil feeding a distributor. Access ranges from lifting a plastic cover to removing an intake manifold on the rear bank of a transverse V6. Cylinder numbering and bank identification vary by make — verify before pulling anything.

What replacement costs

Typically $100–$1,200 at a US independent shop.

Parts and labor at a US independent shop. One accessible coil on a four-cylinder sits near the bottom of the range. A full set on a V6 or V8, especially one requiring intake manifold removal to reach the rear bank, sits near the top. Many shops recommend replacing spark plugs at the same time when access is shared, which is labor-efficient rather than upselling. Individual OEM coils for some makes are notably expensive.

Ranges are wide because access varies enormously between vehicles — the same part can be a twenty-minute job on one engine and a half-day on another.

Trouble codes involving this component