What the Ignition Coil Does in an Engine

The ignition coil converts a car’s 12–14 volts into tens of thousands of volts to fire the spark plugs at precisely the right moment, igniting the air-fuel mixture in gasoline engines; without it, the engine won’t run. In practical terms, the coil acts as a high-voltage transformer and energy reservoir controlled by the engine computer, ensuring reliable combustion across a wide range of loads, temperatures, and altitudes. Note that diesel engines typically don’t use ignition coils because they rely on compression ignition rather than spark.

How an Ignition Coil Works

An ignition coil is a specialized transformer with two windings wrapped around an iron core: a low-voltage primary winding and a high-voltage secondary winding. When the engine control unit (ECU) or an ignition module allows current to flow through the primary winding, a magnetic field builds in the core. At the exact spark timing, the ECU cuts the current. The collapsing magnetic field induces a high voltage in the secondary winding, which travels to the spark plug and jumps the gap, igniting the mixture.

Modern inductive systems typically produce 15–45 kV at the plug depending on gap and cylinder pressure, delivering around 30–100 millijoules of energy with a multi-millisecond spark duration—useful for stable combustion under lean or high-EGR conditions. Capacitive discharge ignition (CDI), used in some performance and small-engine applications, charges a capacitor and dumps it into the coil for a very fast, high-voltage but shorter-duration spark—helpful at very high RPM but sometimes less tolerant of fouled plugs.

Key Components and Signals

The items below outline the core parts of a modern ignition coil system and the signals that control it.

• Primary winding: low-resistance coil that stores energy via magnetic field.
• Secondary winding: many more turns to step up voltage for the spark plug.
• Iron core: concentrates the magnetic field to improve efficiency.
• Boot and insulator: deliver high voltage to the spark plug while preventing arcing.
• Connector and terminals: provide power, ground, and control signals.
• Igniter/driver transistor (in “smart” coils): switches primary current under ECU command.
• ECU control (dwell and timing): sets how long the coil charges and when it fires.
• Noise suppression: internal resistors or ferrites limit radio-frequency interference.

Together, these parts ensure the coil can store energy, generate high voltage on command, and deliver it cleanly to the plug without misfire or electrical noise that could disturb other vehicle systems.

Types of Ignition Coil Systems

Automakers use different coil layouts to balance cost, reliability, packaging, and emissions performance. Here are the main architectures you’ll encounter.

• Single “can” coil with distributor: one coil feeds the distributor, which routes spark to each cylinder mechanically; now rare on modern cars.
• Wasted-spark coil pack: multiple coils fire paired cylinders; simpler than a distributor, common on 1990s–2000s engines.
• Coil-on-plug (COP): one coil per cylinder mounted directly on the plug for maximum voltage at the gap and minimal losses; dominant on modern gasoline engines.
• Coil-near-plug (CNP): coil mounted close to the plug with a short lead; used where packaging or heat is a concern.

COP and CNP systems improve spark reliability, reduce misfire rates, and simplify diagnostics by isolating each cylinder’s ignition path.

What Happens If the Coil Fails

When coils degrade or fail, the engine’s combustion stability suffers. Watch for the following signs.

• Misfires, rough idle, hesitation, or stalling under load or at high RPM.
• Hard starting, especially in damp weather or after a heat soak.
• Poor fuel economy and reduced power.
• Check engine light with codes such as P0300 (random misfire), P0301–P0308 (cylinder-specific misfire), or P0350–P0358 (ignition coil primary/secondary circuit faults).
• Fuel smell from the exhaust, backfiring, or catalyst damage risk if misfires persist.

Because misfires can overheat and poison catalytic converters, it’s best to address ignition issues quickly rather than continuing to drive.

Diagnosis and Testing

Start with an OBD-II scan to retrieve misfire and coil circuit codes, then confirm cylinder-specific issues by moving a suspected coil to a different cylinder and seeing if the misfire follows. A proper ignition spark tester can verify spark strength and consistency; avoid pulling a plug wire on a running engine, which can shock you and damage electronics.

Basic resistance checks of primary and secondary windings can spot open circuits, but many modern failures occur only when hot and under load. An oscilloscope gives the best picture: a healthy primary current ramp typically reaches several amps before cutoff, and the secondary voltage pattern shows the initial firing line and burn time. Always observe high-voltage safety and avoid grounding errors that could harm the ECU.

Maintenance and Replacement Tips

Good spark starts with good fundamentals. The points below can extend coil life and keep your ignition system dependable.

• Replace spark plugs at the recommended interval and use the correct gap; excessive gaps force higher coil voltage.
• Use quality, application-correct coils; bargain units can create radio interference or fail prematurely.
• Replace coil boots and plug seals when brittle or oil-soaked; apply dielectric grease to prevent tracking.
• Keep moisture out of plug wells and fix valve cover leaks that contaminate boots.
• Maintain a healthy battery and charging system; low system voltage can distort dwell and stress coils.
• Check for technical service bulletins (TSBs); some engines have known coil updates or revised plug specs.
• Torque plugs to spec to ensure proper heat transfer and grounding.

Addressing plug condition, sealing, and electrical health reduces stress on coils and prevents repeat failures.

Special Cases and Modern Trends

Diesel engines ignite fuel by compression and generally do not use ignition coils, though they may have glow plugs for cold starts. Hybrids still use conventional ignition coils for their gasoline engines. Modern “smart” COP units integrate the driver transistor and sometimes feedback for diagnostics, improving dwell accuracy and spark control. Direct-injection, turbocharged, and high-compression engines often demand higher spark energy and robust insulation due to elevated cylinder pressures. Some systems employ multi-strike sparking at idle or ion-sensing strategies (notably Saab’s Trionic) to monitor combustion quality directly through the plug.

Summary

The ignition coil is the heart of a gasoline engine’s spark system, transforming low battery voltage into the high voltage and energy needed to ignite the air-fuel mix at precisely the right time. Modern coil-on-plug designs improve reliability and control, while proper maintenance of plugs, boots, and electrical health keeps the system performing. When coils falter, prompt diagnosis and quality replacement protect performance, efficiency, and the catalytic converter.

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.

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 does the engine coil do?

Engine coils (specifically ignition coils) transform the battery’s low-voltage electricity into a high-voltage spark needed to ignite the air-fuel mixture in the engine’s cylinders. They act as transformers, using electromagnetic induction to boost the 12-volt current to thousands of volts. This high-voltage pulse is sent to the spark plugs, creating the spark that initiates combustion and powers the engine.
 
This video explains how ignition coils work and their function in a car’s ignition system: 1mNational MagLabYouTube · Oct 3, 2019
Here’s a breakdown of how they work:

1. Voltage Conversion: Opens in new tabThe ignition coil receives a low-voltage current (around 12 volts) from the car’s battery. 
2. Magnetic Field Generation: Opens in new tabThis low-voltage current passes through a primary coil, creating a magnetic field around an iron core. 
3. High Voltage Induction: Opens in new tabWhen the current is interrupted, the magnetic field rapidly collapses, inducing a much higher voltage in a secondary coil (which has more wire windings than the primary coil). 
4. Spark Plug Ignition: Opens in new tabThis high-voltage electricity is then sent to the spark plugs. 
5. Combustion: Opens in new tabThe spark plugs produce a powerful electric spark that ignites the air-fuel mixture in the engine’s cylinders, causing the engine to run. 

Without a functioning ignition coil, the spark plugs wouldn’t receive the high-voltage energy they need to create a strong enough spark, leading to engine sputtering, misfires, or failure to start.

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.