Ignition Coil Primary/Secondary Circuit

Possible causes

• CommonFailed ignition coil
• CommonWorn or fouled spark plug making the coil work harder until it fails
• CommonCracked coil boot, or oil in the plug well tracking the spark to earth
• CommonCorroded or damaged connector at the coil
• PossibleWiring fault between the module and the coil
• RareFailed control module driver

What you might notice

• Misfire - the engine shaking, especially at idle
• Loss of power and hesitation
• Check engine light on, or flashing
• Poor fuel economy
• Smell of unburnt fuel from the exhaust

What to check, in order

1. Read all codes. A misfire code for a specific cylinder alongside this one identifies which coil to look at first.
2. Swap the suspect coil with one from a neighbouring cylinder and see whether the misfire follows it. This is the single most effective test available and it costs nothing - but only do it on engines where the coils are genuinely interchangeable.
3. Look at the spark plug for that cylinder. A worn plug makes the coil work far harder, and fitting a new coil onto a worn plug often just kills the new coil too.
4. Check the plug well for oil. A leaking cam cover gasket fills the well and the spark tracks to earth through the oil rather than jumping the plug gap.
5. Inspect the coil boot for cracks and carbon tracking.