Ignition Coils Guide
Last updated: 24 April 2026
Ignition coils step up the battery's 12 volts to the 20,000-40,000 volts needed to create a spark at the spark plug. Modern engines use coil-on-plug (COP) designs with one coil per cylinder for precise ignition timing. A faulty coil causes misfires, rough running, increased emissions, and a check engine light. Diagnosing the faulty coil is straightforward with an OBD scanner.
Symptoms of a faulty ignition coil
Ignition coil failure typically affects one cylinder at a time. The engine management system detects the misfire and stores a diagnostic trouble code.
- Check engine light with misfire code (P0301-P0312 for specific cylinders)
- Rough idle — engine shakes or vibrates noticeably
- Loss of power and hesitation under acceleration
- Increased fuel consumption as unburnt fuel passes through the exhaust
- Difficulty starting in cold or damp conditions
Diagnosis and replacement
Coil-on-plug designs make diagnosis easy: swap the suspected coil with an adjacent cylinder's coil. If the misfire follows the coil, that's your faulty unit.
- Read DTCs with an OBD scanner to identify the misfiring cylinder
- Swap coils between cylinders to confirm the fault follows the coil
- Inspect the spark plug in the affected cylinder — it may also need replacing
- Check the coil connector for corrosion or damaged pins
- If one coil has failed and others are original high-mileage units, consider replacing the full set
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Misfires and rough idle? How to identify a faulty coil with an OBD scanner and decide on replacement.
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Frequently Asked Questions
What does an ignition coil do?
It transforms the battery's 12V to the 20,000-40,000V needed to create a spark at the spark plug, igniting the fuel-air mixture in the cylinder.
How do I know which ignition coil is faulty?
Read the diagnostic trouble code (DTC) with an OBD scanner — it will identify the misfiring cylinder. Swap coils between cylinders to confirm the fault follows the coil.
Should I replace all ignition coils at once?
Not necessarily. Replace the faulty one first. If multiple coils are original and high-mileage, replacing the set prevents repeat failures.
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