What Is an Ignition Coil?
The ignition coil is an electromagnetic transformer that converts the car's 12-volt battery supply into the high-voltage pulse needed to fire the spark plugs. A typical spark plug requires anywhere from 12,000 to 45,000 volts to jump its electrode gap — the ignition coil is what makes that possible.
Modern petrol vehicles use one of two main configurations:
- Coil-on-plug (COP) — one coil per cylinder, mounted directly on top of the spark plug. Most common on vehicles built after 2000
- Distributor-based coil — a single coil feeds all cylinders via a distributor rotor and cap (older, pre-2000 vehicles)
- Coil pack — one coil for every two cylinders, an intermediate system common on 1990s–2000s vehicles
How Does an Ignition Coil Work?
Inside the coil, two windings of copper wire — a primary winding (fewer turns, thicker wire) and a secondary winding (many more turns, fine wire) — are wrapped around an iron core.
When the ECU signals the coil to fire, it interrupts the current in the primary winding. This causes a rapidly collapsing magnetic field, which induces a very high voltage in the secondary winding (by Faraday's law of electromagnetic induction). This high voltage spike travels to the spark plug and jumps the gap, creating the ignition spark.
The ECU controls the precise timing of this spark — advanced or retarded depending on engine load, speed, and temperature — to optimise power and fuel economy.
Signs of a Failing Ignition Coil
| Symptom | Notes |
|---|---|
| Engine misfire (rough running, shaking) | Cylinder with failed coil stops firing |
| Check Engine light (misfire codes) | P0300 (random) or P030X (specific cylinder) |
| Loss of power | Reduced number of firing cylinders |
| Poor fuel economy | Unburnt fuel passing through to exhaust |
| Hard starting | Weak or no spark |
| Backfiring | Unburnt fuel igniting in exhaust |
| Stalling | Particularly at idle with multiple coil failure |
On a COP system, a failed coil means exactly one cylinder is not firing. The remaining cylinders continue normally, but the engine will run very roughly and the catalytic converter will be damaged by the unburnt fuel flowing through it.
Diagnosing an Ignition Coil Failure
Mechanics use an OBD-II scan tool to read misfire codes. A code like P0302 (cylinder 2 misfire) points directly to the coil or spark plug on that cylinder. The quickest test is to swap the suspect coil with a known-good coil from another cylinder — if the misfire code moves to the new cylinder, the coil is at fault.
This is one reason why COP systems are excellent for diagnosis — each coil is individual and can be tested or swapped independently.
What Happens If You Ignore a Failed Coil?
- Misfires send raw petrol through the catalytic converter, overheating it and destroying the ceramic substrate — a $500–$1,500 repair
- Engine vibration from a cylinder that's not firing stresses engine mounts
- Fouled spark plugs in the affected cylinder
- Increased fuel consumption as the ECU adds fuel to compensate
Don't drive for extended periods with a known misfire. It's a secondary-damage scenario.
NZ Repair Cost Estimates
| Repair | Typical NZ Cost |
|---|---|
| Single COP coil replacement | $100–$280 fitted |
| Full set of coils (4-cylinder) | $300–$700 fitted |
| Coil pack replacement | $180–$400 fitted |
| Distributor cap and rotor (older vehicles) | $100–$200 |
Coil prices vary widely by manufacturer. Japanese OEM coils (Denso, NGK) for Toyota, Mazda, Honda, and Subaru are generally well-priced in NZ and highly reliable. Some European brands (BMW, Mercedes, Audi) have coils that cost significantly more.
When to Book a Mechanic
- Check Engine light is on with misfire codes
- The engine shakes noticeably, especially at idle
- Hard starting has become the norm
- You notice a loss of power that wasn't there before
Get a scan done promptly — misfire damage to the catalytic converter is cumulative and expensive.