An ignition coil is a compact transformer that turns your battery's 12 volts into the 20,000 to 45,000 volts needed to jump a spark plug gap. It stores energy in a magnetic field, then releases it in an instant when the engine computer triggers the spark.
Animated: how a Ignition Coil actually works
🔧 How It Works, Step by Step
1
Energize the primary
The ECM feeds 12 volts through the coil's primary winding, building a strong magnetic field.
2
Store magnetic energy
Current flows for a few milliseconds, charging the iron core with magnetic energy.
3
Interrupt the current
The ECM suddenly switches off the primary current at the exact moment of ignition.
4
Induce high voltage
The collapsing field induces a huge voltage in the thousands of turns of the secondary winding.
5
Fire the plug
That high voltage travels to the spark plug and arcs across the gap to ignite the mixture.
🧩 The Key Parts
Primary winding
Few hundred turns that carry the 12-volt charging current.
Secondary winding
Thousands of turns that produce the high output voltage.
Iron core
Concentrates the magnetic field to boost energy transfer.
High-tension terminal
Delivers the high voltage to the spark plug or wire.
Ignitor circuit
Electronic switch that times the current interruption.
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🩺 Signs of a Failing Ignition Coil
Cylinder misfire or rough running
Flashing or steady check engine light
Hard starting, especially when hot
Loss of power and hesitation
Poor fuel economy
Engine stalling at idle
⚠️ Common Problems
Insulation breakdown
Heat and vibration crack the coil's insulation, letting voltage leak instead of reaching the plug.
Open or shorted winding
A broken or shorted winding stops the coil from producing a usable spark.
Heat fatigue
Underhood heat cycling degrades coil-on-plug units, causing intermittent misfires that worsen over time.
💰 Cost to Fix
$100-$350typical range to repair or replace, parts and labor
❓ FAQ
Do all coils fail at once?
No. Coil-on-plug systems use one coil per cylinder, so usually just one fails, but replacing them as a set is common on high-mileage engines.
Can a bad coil damage the engine?
A persistent misfire can dump raw fuel into the catalytic converter and overheat it, so a failed coil should be fixed promptly.
How long do ignition coils last?
Many last 100,000 miles or more, but heat, oil leaks, and worn plugs can shorten their life.