What an Ignition Coil Does in a Car

An ignition coil converts the car’s 12-volt battery power into tens of thousands of volts—often 20,000 to 60,000 volts—so the spark plugs can ignite the air-fuel mixture in the engine’s cylinders. In modern vehicles, the engine computer precisely controls each coil’s timing and energy to deliver reliable sparks for efficient combustion, performance, and emissions control.

How the Ignition Coil Works

At its core, an ignition coil is a small transformer with two windings that stores energy in a magnetic field and then releases it as a high-voltage pulse. This pulse jumps the spark plug gap, igniting the mixture exactly when the engine needs it.

The following sequence outlines how a typical coil produces spark:

1. Primary charging: The engine control module (ECM) grounds the coil’s primary circuit, allowing current from the 12V battery to build a magnetic field in the coil.
2. Dwell control: The ECM holds the current for a calculated “dwell” time, optimized for battery voltage, engine speed, and load.
3. Field collapse: The ECM then opens the primary circuit; the magnetic field collapses rapidly across the secondary winding.
4. Voltage step-up: The collapsing field induces a high voltage in the secondary winding, creating a pulse capable of jumping the spark plug gap.
5. Spark event: The voltage travels through the plug wire or direct connector to the spark plug, creating a spark that ignites the air-fuel mixture.

This controlled charge-and-collapse cycle happens thousands of times per minute, synchronized to crankshaft and camshaft position signals for accurate ignition timing.

Common Ignition Coil Arrangements

Automakers use several configurations to deliver high voltage to spark plugs, balancing cost, reliability, packaging, and serviceability.

• Coil-on-plug (COP): One coil per cylinder mounted directly on each spark plug. Offers precise control and eliminates plug wires; common on most modern engines.
• Coil-near-plug: Similar to COP but with a short lead to the plug; used where packaging is tight or heat is a concern.
• Coil pack (wasted spark): A pack of coils firing pairs of cylinders; simpler and cheaper than COP, still common on some engines.
• Distributor-based single coil (older designs): One coil sends high voltage through a mechanical distributor to all plugs; largely phased out due to wear and timing limitations.

While all designs transform voltage, newer coil-per-cylinder systems improve reliability, spark precision, and emissions by letting the ECM tailor energy and timing per cylinder.

The Coil’s Role in Modern Engine Management

Beyond simple spark, today’s coils support strategies that reduce emissions, prevent knock, and improve fuel economy. The ECM adjusts dwell to maintain spark energy at high RPM, retards or advances timing based on knock sensor feedback, and may command multiple sparks at idle or during cold starts. Turbocharged and direct-injection engines often demand higher-energy sparks to ignite denser, stratified mixtures—placing premium on coil health and thermal management.

Symptoms of a Failing Ignition Coil

Because coils are critical to combustion, faults typically produce noticeable drivability issues and fault codes. Early detection can prevent catalytic converter damage and poor fuel economy.

• Misfires, rough idle, or hesitation—especially under load or during acceleration.
• Hard starts or stalling, particularly when hot.
• Check Engine Light with codes such as P0300 (random misfire) or cylinder-specific P0301–P0308; coil circuit codes like P0351–P0362 indicate primary/secondary faults.
• Reduced power, poor fuel economy, and increased tailpipe emissions.
• Audible “snapping” or visible arcing in dark conditions (on plug wires or boots), and fuel smell from unburned mixture.

If these signs appear, testing the suspect coil(s) before continued driving can avoid downstream damage and higher repair costs.

Why Ignition Coils Fail—and How to Prevent It

Coils endure heat, vibration, and high voltage. Over time, insulation and internal components can degrade. Some external conditions accelerate failure.

• Heat and vibration: Engine bay temperatures and physical shock stress windings and resin potting.
• Oil or coolant contamination: Leaks into plug wells attack coil boots and insulation.
• Worn spark plugs or excessive plug gaps: Force coils to work harder, raising voltage demands and heat.
• Moisture and corrosion: Water ingress, especially after washing or in humid climates, can promote tracking and misfires.
• Poor electrical grounds or low system voltage: Reduce dwell control accuracy and increase stress.

Addressing leaks, replacing spark plugs at the recommended intervals, and ensuring good grounds are straightforward steps that extend coil life.

Maintenance and Testing Tips

Basic checks can confirm a coil problem and help distinguish it from injector, plug, or wiring faults.

• Scan for codes and monitor misfire counters to identify affected cylinders.
• Swap test: Move a suspect COP coil to another cylinder; if the misfire follows, the coil is likely faulty.
• Inspect plugs and boots: Look for oil, carbon tracking, cracks, or burn marks; replace damaged boots with dielectric grease on reassembly.
• Measure primary/secondary resistance (when specified by the manufacturer), but note that many failures only show up under load; an oscilloscope offers the most definitive diagnosis.
• Check battery health and charging voltage; weak supply can mimic ignition issues.

Always disable the ignition and follow the vehicle service manual when testing. Coils can generate dangerous voltages—exercise caution.

Replacement Considerations and Costs

Replacing coils ranges from simple to time-consuming depending on engine layout. Many technicians recommend replacing spark plugs and boots with coils to prevent repeat visits.

• Typical parts cost: $30–$120 per coil for mainstream vehicles; $150–$300+ for some premium or performance models.
• Labor: 0.3–1.5 hours per coil in accessible COP setups; more if intake components must be removed.
• Best practice: Replace the boot and plug with the coil; on high-mileage engines with repeated failures, consider replacing the full set.
• Warranty: Quality aftermarket or OEM coils often carry 1–3 year warranties; verify coverage and return policies.

Using OEM or reputable aftermarket coils helps ensure correct connector fit, dwell behavior, and durability, especially on engines sensitive to ignition performance.

Safety Note

Ignition systems can produce lethal voltages. Do not probe a live secondary circuit. Disconnect the battery, follow manufacturer procedures, and use insulated tools and protective equipment during diagnosis and repair.

Summary

The ignition coil is a high-voltage transformer that turns a car’s 12 volts into a powerful pulse to fire each spark plug at precisely the right moment. Modern coil designs—especially coil-on-plug—work under tight electronic control to maximize efficiency, performance, and emissions compliance. Watch for misfires, warning lights, and rough running as signs of coil trouble, and protect coils by fixing leaks and maintaining spark plugs on schedule.

What happens to a car when the ignition 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 much does it cost to fix an ignition coil?

Replacing an ignition coil typically costs between $230 and $330, though this can vary significantly based on your vehicle’s make and model, the cost of the specific coil needed, and regional labor rates. High-end or specialty vehicles may have more expensive parts and labor, while DIY replacement can lower the cost substantially for those with mechanical skills. 
Factors influencing cost

• Vehicle Make and Model: Different cars require different types of ignition coils, which impacts price. 
• Type of Coil: More technologically advanced ignition systems, such as coil-on-plug (COP) systems, can have more expensive components. 
• Labor vs. Parts: The total cost includes parts and labor, which can vary by location. 
• Diagnosis Fee: Some shops may charge an additional fee for diagnosing the issue, even before starting the repair. 
• Additional Parts: If the diagnostic reveals that spark plugs also need replacing, the total cost will increase. 

Understanding Ignition Coil Replacement

• No “Fixing”: Opens in new tabIgnition coils are often considered disposable parts; they are not typically repaired but replaced. 
• DIY Option: Opens in new tabIf you are comfortable working on your vehicle, you can purchase the necessary parts yourself and perform the replacement after watching some YouTube tutorials, which can significantly reduce the cost. 
• Professional Repair: Opens in new tabFor a professional repair, you can expect to pay more. To get a better estimate, you may need to provide the mechanic with your vehicle’s VIN and confirm the hourly labor rate at the shop. 
• Comparison Shopping: Opens in new tabIt is often a good idea to get quotes from multiple mechanics or shops, as prices can vary. 

What are the symptoms of a failing ignition coil?

Symptoms of a failing ignition coil include a Check Engine Light, engine misfires and rough idling, stalling, loss of power, poor fuel economy, difficulty starting the engine, and exhaust backfiring or a smell of gas. These issues occur because the coil fails to deliver enough voltage to the spark plugs, preventing proper combustion. 
Common Symptoms

• Check Engine Light: A blinking or steady Check Engine light often indicates a misfire, which is a common sign of a failing ignition coil. 
• Engine Misfires/Rough Idling: You may notice a jerky or stuttering motion while driving, or the engine may feel unstable and vibrate when idling. 
• Stalling: The engine might cut off or stall when you come to a stop. 
• Loss of Power: The vehicle may feel sluggish, especially during acceleration, due to incomplete combustion in the cylinders. 
• Poor Fuel Economy: With less efficient combustion, your car will consume more fuel, leading to lower gas mileage. 
• Difficulty Starting: The engine may crank longer or fail to start altogether. 
• Exhaust Backfiring: You might hear a sputtering or coughing sound from the engine or tailpipe. 
• Smell of Gas: An incomplete combustion process can send unignited fuel into the exhaust, creating a gas smell. 

What’s Happening Inside the Engine
A failing ignition coil cannot provide the proper voltage to the spark plug. This leads to: 

• Incomplete Combustion: The fuel-air mixture in the cylinder doesn’t ignite correctly. 
• Misfires: The spark plug fails to do its job, causing the engine to misfire. 
• Engine Performance Issues: The misfires disrupt the smooth operation of the engine, resulting in the symptoms listed above. 

How do you know if you need an ignition coil for your car?

6 Signs of a Faulty Ignition Coil

1. Faltering Engine Power. A fully functioning engine needs all cylinders firing happily away.
2. Misfiring Cylinders.
3. Rough Idling.
4. Poor Gas Mileage.
5. Exhaust Problems.
6. Check Engine Light.
7. How Frequently To Replace Coils at Your Dealer.