What an Engine’s Coil Does — and Why It Matters

The coil in a gasoline engine—commonly the ignition coil—converts the vehicle’s low 12-volt battery power into tens of thousands of volts to fire the spark plugs at precisely the right moment, igniting the air-fuel mixture. Without a healthy coil, the engine will misfire, run poorly, or fail to start. While “coil” can also refer to other electromagnetic windings in a vehicle, such as alternator stator coils or solenoid coils, the ignition coil is the one most people mean when asking what a coil does in an engine.

How an Ignition Coil Works

At its core, an ignition coil is a step-up transformer with two windings: a primary winding that builds a magnetic field when energized, and a secondary winding that generates high voltage when that field collapses. Modern engine control modules (ECMs) switch the coil on and off rapidly, controlling “dwell time” to store energy and then release it as a high-voltage pulse—often 20,000 to 40,000 volts, and in some systems beyond 60,000 volts—to jump the spark plug gap and ignite the mixture. Coil energy delivery and timing are synchronized with crank and cam position signals to ensure efficient combustion across the rev range.

From Distributor to Coil-On-Plug

Older vehicles often used a single coil feeding a distributor, which mechanically routed spark to each cylinder. Most modern engines use coil packs (serving multiple cylinders) or coil-on-plug (COP) units with one coil per cylinder. COP designs reduce energy losses, improve timing precision, help meet emissions targets, and simplify high-voltage routing by placing the coil directly on the spark plug.

Types of “Coils” You Might Mean

When drivers say “the coil,” they usually mean the ignition coil. But vehicles contain several different coils with distinct roles. The following list explains the most common ones you may encounter.

• Ignition coil (single, coil pack, or coil-on-plug): Steps up 12 V to high voltage to fire spark plugs in gasoline engines.
• Pickup coil (older systems): A sensor coil in distributors that helped trigger ignition timing before widespread crank/cam sensors.
• Alternator stator coils: Generate AC power that’s rectified to DC to charge the battery and run electrical systems.
• Solenoid and actuator coils: Electromagnetic windings in components like fuel injectors, purge valves, variable valve timing (VVT) actuators, and starter solenoids.
• Motor/generator stator coils (hybrids/EVs): Produce magnetic fields for propulsion and regenerative braking—these vehicles typically have no ignition coils.

Knowing which coil is in question helps clarify symptoms and repair paths. In everyday gasoline car discussions, it’s almost always the ignition coil.

Symptoms of a Failing Ignition Coil

Because ignition coils are central to spark generation, their failures can mimic a range of engine problems. Watch for the indicators below to catch issues early.

• Misfires, rough idle, hesitation under load, or stalling—often worse in wet or hot conditions.
• Hard starting or no-start, especially if multiple coils or a single-coil distributor system fails.
• Poor fuel economy and reduced power, as incomplete combustion wastes fuel.
• Check Engine Light with misfire codes (P0300–P030X) or coil circuit codes (P0351–P0358 on many vehicles).
• Raw fuel smell, backfiring, or catalytic converter damage from prolonged misfires.

If these symptoms appear, timely diagnosis matters; running with a weak coil can quickly damage spark plugs and catalytic converters.

Common Causes and How Technicians Diagnose

Coils live in harsh environments, contending with heat, vibration, and moisture. Failures often trace to insulation breakdown, cracked housings or boots, oil intrusion from leaky valve covers, or excessive spark plug gap that over-stresses the coil. Technicians use several approaches to pinpoint the fault efficiently.

1. Scan tool checks: Confirm misfire counters and coil control faults; look at fuel trims and related sensor data.
2. Swap test: Move a suspect coil to another cylinder and see if the misfire follows.
3. Scope testing: Examine coil primary current ramp and secondary voltage patterns to assess dwell, saturation, and spark duration.
4. Resistance and insulation checks: Measure primary/secondary resistance and inspect for carbon tracking or moisture inside boots.
5. Spark plug review: Verify correct plug type and gap; worn plugs increase coil stress and heat.

Because ignition systems operate at dangerous voltages, avoid pulling coils or wires off live cylinders; use insulated tools and proper diagnostic equipment.

Maintenance and Replacement Tips

Good ignition health starts with correct parts and attention to detail. These practical steps help extend coil life and maintain strong spark.

• Use manufacturer-specified spark plugs and maintain proper gap; replace at recommended intervals.
• When replacing COP units, consider new boots and apply a small amount of dielectric grease inside the boot to prevent tracking.
• Torque spark plugs correctly to ensure heat transfer and avoid damaging threads or insulators.
• Address oil leaks promptly—oil in plug wells degrades coil boots and insulation.
• Stick with reputable coil brands; ultra-cheap coils often have inferior insulation and shorter lifespans.

Preventive maintenance reduces misfires, protects catalytic converters, and minimizes surprise no-starts tied to ignition faults.

Other Engine and Vehicle Coils Worth Noting

Diesel engines typically don’t use ignition coils because they rely on compression ignition; instead, they may have glow plugs to aid cold starts. On the electrical side, alternator stator coils keep the 12-volt system charged, while hybrids and EVs use large motor/generator coils for propulsion rather than spark generation. Understanding these differences helps avoid confusion when discussing coil-related problems across different powertrains.

Bottom Line

An ignition coil’s job is to convert low battery voltage into a precisely timed, high-voltage pulse that ignites the air-fuel mixture in a gasoline engine. Modern coil-on-plug systems deliver cleaner, more reliable spark, improving performance and emissions. When coils fail, misfires and drivability issues follow—diagnose quickly and maintain the ignition system to keep engines running smoothly.

How much does it cost to replace an engine coil?

Replacing an engine ignition coil typically costs between $150 and $600 for a single coil, with the average falling around $230 to $330, though costs vary significantly based on your vehicle’s make and model. This total includes both the price of the coil itself, which can range from $50 to $300 or more, and labor costs, usually billed at $50 to $150 per hour. More complex or luxury vehicle parts and labor at a dealership will increase the overall price. 
Cost Breakdown

• Parts: A single ignition coil can cost anywhere from $50 to over $300, depending on your car’s make and model. 
• Labor: Labor charges for replacement are typically $50 to $150 per hour, with the total time ranging from 0.5 to 2 hours depending on the vehicle and coil location. 

Factors Influencing the Cost

• Vehicle Type: Imported vehicles, especially Japanese models, often have higher part and labor costs than domestic vehicles. 
• Coil Type: Newer coil-on-plug (COP) systems can be more expensive to replace than older, conventional ignition systems. 
• Location: Dealerships generally charge higher rates for both parts and labor than independent repair shops. 
• Coil Location: Coils located in harder-to-reach areas or requiring other parts to be removed will increase labor time and cost. 
• OEM vs. Aftermarket: OEM (original equipment manufacturer) parts tend to be more expensive but are often recommended for their longevity. 

When to Seek Professional Help
If you are unsure about the cause of your engine problem, it’s best to have a skilled mechanic diagnose the issue, according to Synchrony. A qualified professional can accurately identify the faulty coil and perform the replacement, saving you potential further damage and ensuring the job is done correctly.

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.

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.

Will a car run without an ignition coil?

Without a functioning ignition coil, your engine won’t start, or it may misfire and run poorly.