What the Engine Coil Does

The engine (ignition) coil transforms the car’s 12-volt battery power into tens of thousands of volts to jump the spark plug gap and ignite the air-fuel mixture; without it, a gasoline engine won’t run. In practical terms, the coil is the heart of the spark-ignition system, controlled by the engine computer to deliver a precisely timed, high-energy spark for efficient combustion, performance, and emissions control.

What the Coil’s Job Is in a Modern Engine

On gasoline engines, the coil supplies the high voltage needed to fire each spark plug right when the piston approaches the top of its compression stroke. The engine control unit (ECU) or ignition module charges the coil and releases it at the exact moment based on crankshaft/camshaft position sensors, throttle position, load, and knock feedback. This timing and energy delivery directly influence power, fuel economy, and catalytic converter health. Diesel engines, by contrast, ignite fuel by compression and typically do not use ignition coils.

How an Ignition Coil Works

An ignition coil is a transformer with two windings. The primary winding builds a magnetic field as current flows; when that current is abruptly interrupted, the collapsing field induces a very high voltage in the secondary winding. That voltage—often 20,000 to 40,000 volts, and sometimes higher—travels through the coil boot to jump the spark plug gap, igniting the mixture.

Key stages in coil operation

The following points break down what happens each time the coil fires.

• Dwell/charge: The ECU or ignition module switches the coil on, allowing current to ramp up in the primary winding.
• Field collapse: The switch opens; the magnetic field collapses rapidly.
• High-voltage induction: The collapse induces a high voltage in the secondary winding.
• Spark delivery: Voltage travels through the boot/wire to the spark plug, overcoming the gap and generating a spark.
• Burn time: The spark lasts milliseconds, sustaining combustion across the plug gap.

Together, these steps repeat for each cylinder at thousands of times per minute, with the ECU adjusting dwell time and timing to match engine conditions and protect components.

Common Coil Designs You’ll See

Automakers use several coil configurations, selected for packaging, durability, and performance needs.

• Single canister + distributor: One coil feeds a distributor that routes spark to all cylinders (older systems).
• DIS “waste-spark” coil packs: Multiple coils fire pairs of cylinders; simpler than distributors, common in the 1990s–2000s.
• Coil-on-plug (COP): One coil per spark plug; minimizes losses, improves control, now standard on most late-model cars.
• Coil-near-plug: Coils sit close to plugs and connect via short leads; used where packaging demands.
• Capacitive discharge (CDI) coils: Common in powersports/rotary engines; store energy in a capacitor for very fast, high-energy sparks.

While all designs serve the same purpose, COP systems dominate current vehicles for better reliability, emissions, and misfire control.

Symptoms of a Failing Ignition Coil

When coils weaken or fail, the engine’s spark quality drops, especially under load. Drivers and technicians often notice the following.

• Misfires, rough idle, hesitation, or shudder during acceleration.
• Hard starting or stalling, particularly when hot or in wet weather.
• Poor fuel economy and a raw-fuel smell from the exhaust.
• Check engine light; codes like P0300 (random misfire), P0301–P0308 (cylinder-specific), or P0351–P0362 (coil primary/secondary faults).
• Catalytic converter damage risk if misfires persist (flashing MIL indicates immediate attention needed).
• Visible cracking, carbon tracking on the coil or boot, or oil in spark plug wells.

Because symptoms overlap with bad plugs, wires, injectors, or compression issues, testing is essential before replacing parts.

Why Coils Fail

Coils are robust but live in a harsh environment. These stressors commonly cause breakdowns.

• Heat and vibration that degrade insulation and windings.
• Over-gapped or worn spark plugs increasing voltage demand.
• Moisture intrusion, oil contamination from valve cover leaks, or coolant leaks.
• High resistance in plug wires/boots or carbon tracking causing arcing.
• Poor grounds or charging system issues affecting coil current control.
• Incorrect dwell from a failing driver/module, or low-quality replacement parts.

Addressing root causes—especially plug condition and sealing leaks—prevents repeat failures and protects the catalytic converter.

How to Diagnose an Ignition Coil Safely

High voltage from coils can be dangerous. Avoid pulling plug wires on a running engine; use proper tools and procedures instead.

1. Scan for codes and misfire counters; note which cylinder(s) misfire and under what conditions.
2. Inspect coils, boots, and plug wells for cracks, carbon tracks, oil, or water.
3. Check spark plugs for wear, fouling, cracks, and correct gap; replace if due.
4. Perform a “swap test” on COP systems: move a suspect coil to another cylinder and see if the misfire follows.
5. Use an inline adjustable spark tester to confirm spark strength without risking the ECU or yourself.
6. Measure coil primary current ramp with an amp clamp and scope (professional method) to check saturation and dwell.
7. Verify grounds and charging voltage; poor voltage control can mimic coil faults.
8. If multiple cylinders misfire, consider upstream issues: vacuum leaks, fuel pressure, injector control, or engine mechanical problems.

These steps help isolate a bad coil from other ignition or engine issues, reducing guesswork and unnecessary parts replacement.

Maintenance and Prevention Tips

Keeping the ignition system healthy extends coil life and protects the catalytic converter.

• Replace spark plugs at the recommended interval and use the specified type and gap.
• Install new boots or wires when replacing plugs if wear is evident; apply dielectric grease at the boot tip.
• Fix valve cover gasket leaks and keep plug wells dry and clean.
• Ensure solid engine and battery grounds; maintain a healthy charging system.
• Use quality coils from reputable brands, especially on turbocharged or high-compression engines.
• Avoid pressure-washing the engine; if you must, shield ignition components.

Routine maintenance cuts misfires, improves drivability, and reduces the chance of sudden coil failure.

Costs and Replacement Choices

Prices vary by vehicle and coil design, but typical ranges are predictable.

• Coil-on-plug units: roughly $40–$150 each for parts; labor often 0.3–1.0 hour per coil depending on access.
• Coil packs (waste-spark): commonly $100–$400 for the pack; labor 0.5–1.0 hour.
• Related service: new spark plugs, boots/wires, and fixing oil leaks may add cost but can prevent repeat failures.
• Replacement strategy: replace only the failed coil unless multiple are aging or failing in quick succession; always evaluate plugs/boots at the same time.

Combining coil replacement with overdue plug service often restores performance and reduces future shop visits.

What About Diesel Engines?

Diesels don’t use ignition coils because they ignite fuel by high compression. Many diesels use glow plugs and controllers to aid cold starts, which are separate heating elements—not ignition coils. If someone refers to a “coil” on a diesel, they may mean a glow plug relay or a coil in a fuel solenoid, not an ignition coil.

Summary

The engine (ignition) coil is the high-voltage generator that fires the spark plugs in gasoline engines, converting 12 volts into the energy needed for ignition at precisely timed intervals. Most modern vehicles use one coil per plug for reliability and control. Misfires, hard starts, and check-engine lights often point to coil or related ignition issues, but proper diagnosis—checking plugs, boots, wiring, and voltage supply—is key. With routine maintenance and quality parts, coils typically deliver long, trouble-free service.

Is it expensive to replace an engine coil?

If you have a faulty coil, it’s going to cost you between $200 and $300 on average to replace it, not to mention the frustration and downtime you’ll experience. Learn more about what an ignition coil is, what it does, ignition coil replacement cost, and how you can tell when it’s bad.

How to tell if a motor coil is bad?

6 Signs of a Faulty Ignition Coil

1. Signs of a Faulty Ignition Coil: Check Engine Light.
2. The Engine Starts Misfiring.
3. The Engine Won’t Start Normally.
4. The Engine Guzzles Gas.
5. The Car Doesn’t Perform Well.
6. The Car Backfires.

What happens when an engine coil goes bad?

One of the most common symptoms associated with a faulty ignition coil is engine performance issues… Faulty coils may cause the vehicle to experience misfires, a rough idle, a loss in power and acceleration, and a reduction in gas mileage. In some cases the performance issues may even result in the vehicle stalling.

Can I drive with a damaged coil?

Driving a car with a faulty coil is not advisable, as it can cause the engine to misfire and potentially damage the catalytic converter.