What the Ignition Coil Does in a Car

The ignition coil converts a car’s 12-volt battery power into the high voltage (often 15,000–40,000+ volts) needed to fire the spark plugs, igniting the air-fuel mixture in gasoline engines. It acts as a transformer controlled by the engine computer or ignition module to deliver a precisely timed spark under varying engine conditions.

How the Ignition Coil Works

At its core, the ignition coil is a step-up transformer with two windings: a primary coil that builds a magnetic field when energized, and a secondary coil that generates a high-voltage pulse when that field collapses. Modern engine control units (ECUs) rapidly switch the primary circuit on and off (controlling “dwell” time) to produce strong, precisely timed sparks for efficient combustion.

Key Components and Operation

The following items summarize the parts and physics that make an ignition coil function and how they interact during each firing event.

• Primary winding: Low-resistance coil that builds a magnetic field when supplied with 12V.
• Secondary winding: High-turn-count coil that induces high voltage when the primary field collapses.
• Ignition driver: A transistor (in the ECU or built into the coil) that switches the primary current.
• Dwell control: ECU-managed time the primary circuit is energized to optimize spark energy.
• Spark plug interface: High voltage arcs across the plug gap to ignite the mixture.
• Timing input: Crankshaft/camshaft sensors tell the ECU when to trigger each coil.

Together, these elements transform low-voltage battery power into a controlled, high-energy spark that happens at the exact crank angle the engine needs for power and efficiency.

Common Coil System Designs

Automakers use several coil configurations, each balancing simplicity, reliability, and serviceability. Understanding the layout helps with diagnosis and parts selection.

• Coil-on-plug (COP): One coil per cylinder mounted directly on each plug; most common on modern engines for precise control and minimal energy loss.
• Coil-near-plug (CNP): Individual coils mounted near (but not on) the plugs, connected by short wires.
• Wasted-spark coil packs: One coil fires two cylinders at once, one on the compression stroke and one on the exhaust stroke (“wasted” spark).
• Single coil with distributor (older systems): One central coil sends high voltage via a distributor rotor and cap to each plug.

While COP systems dominate current designs due to accuracy and emissions benefits, coil packs and wasted-spark setups remain common on slightly older vehicles and some cost-sensitive applications.

What Happens If an Ignition Coil Fails

A failing coil weakens or eliminates spark at one or more cylinders, causing noticeable drivability issues and potential long-term damage if ignored.

• Misfires, rough idle, hesitation, or lack of power under load.
• Poor fuel economy and increased emissions; potential check-engine light.
• Hard starting or no-start (especially with single-coil/distributor systems).
• Raw fuel in the exhaust can overheat/damage the catalytic converter.
• Intermittent problems that worsen with heat (thermal breakdown).

Because misfires can quickly harm the catalytic converter, addressing coil issues promptly can prevent costly repairs.

Why Ignition Coils Fail

Coils endure heat, vibration, and high voltage. Several underlying issues can accelerate failure and should be checked during diagnosis.

• Heat and vibration causing insulation breakdown or cracked housings.
• Oil or coolant intrusion into the plug well (from leaking valve cover gaskets or coolant passages).
• Excessive spark plug gap or worn plugs forcing the coil to work harder.
• Moisture ingress and corrosion at connectors or boots.
• Voltage spikes from weak battery/alternator or poor grounds.
• Inferior aftermarket parts with weak insulation or poor winding quality.

Correcting root causes—especially plug wear and fluid leaks—extends coil life and improves reliability.

Diagnosing Ignition Coil Problems

Systematic checks help distinguish a bad coil from issues like injectors, compression, or fuel delivery. Start with codes and simple tests, then move to targeted measurements.

1. Scan for OBD-II codes: Misfire codes (P0300–P0308) and coil circuit codes (P0350–P0362) point to affected cylinders.
2. Swap test (COP systems): Move a suspected coil to another cylinder and see if the misfire follows.
3. Inspect spark plugs and boots: Look for fouling, cracks, tracking, incorrect gap, or oil/coolant.
4. Check power, ground, and trigger: Verify 12V supply, ground integrity, and ECU trigger with a multimeter or oscilloscope.
5. Scope testing: Use an ignition oscilloscope to view primary/secondary waveforms and dwell; look for weak or irregular patterns.
6. Resistance checks (older coils): Measure primary/secondary resistance; note that many modern coils don’t reliably diagnose by resistance alone.
7. Spray test for intermittents: Lightly mist water on boots/wires (engine idling) to reveal arcing—use caution around high voltage.

If tests confirm a coil fault, verify there isn’t a contributing issue (plug gap, leaks, wiring) before installing a new part.

Repair and Replacement Tips

Proper parts selection and installation techniques improve longevity and prevent repeat failures.

• Replace spark plugs with the correct type and gap when replacing coils, especially on high-mileage engines.
• Address oil or coolant leaks into plug wells before installing new coils.
• Use dielectric grease on the inside of boots to reduce tracking and ease future removal.
• Torque plugs and coils to spec to avoid thread damage and ensure good heat transfer.
• Stick with quality OEM or reputable aftermarket coils; avoid mixed brands across cylinders on sensitive engines.
• Clear codes and perform a drive cycle to confirm the fix and readiness monitors.

These steps help ensure a durable repair and stable ignition performance across all cylinders.

Safety Considerations

Ignition systems can produce tens of thousands of volts; even brief contact can be dangerous. Work carefully and follow basic precautions.

• Never touch ignition components with the engine running without proper insulation.
• Disconnect the battery when servicing primary wiring; keep tools dry and insulated.
• Avoid pulling coils or wires off running engines to “check for spark”; use proper testers.
• Beware of rotating components and hot surfaces while diagnosing.

With the right tools and techniques, ignition work can be done safely and effectively.

Additional Context

Ignition coils are used only in spark-ignition engines (gasoline, some alternative fuels). Diesel engines typically rely on compression ignition and do not use spark plugs or ignition coils. On many modern vehicles, the ECU contains the coil drivers, though some coils integrate their own drivers for faster, cooler operation and improved reliability.

Performance and Emissions Impact

A healthy ignition coil ensures complete combustion, maximizing power, drivability, and fuel economy while minimizing emissions. Weak coils can cause high HC emissions, rough running, and catalytic converter damage—often far more expensive than the coil itself.

Summary

The ignition coil is a step-up transformer that turns 12V battery power into the high-voltage pulse that fires each spark plug at precisely the right moment. Modern systems use coil-on-plug or coil packs controlled by the ECU for accurate timing and strong spark energy. Symptoms of failure include misfires, rough running, poor economy, and potential catalytic damage. Proper diagnosis involves scanning for codes, inspection, electrical checks, and sometimes oscilloscope testing. Replacing worn plugs, fixing leaks, and using quality parts are key to a reliable, long-lasting repair.

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 should it cost to replace an ignition coil?

An ignition coil replacement generally costs between $230 and $330, but this can vary significantly based on the vehicle’s make and model, the specific type of ignition coil (e.g., conventional vs. coil-on-plug), and whether it’s a DIY job or professional service. The total cost includes the price of the part itself, which can range from about $50 to $600, and labor costs, which often range from $75 to $150 per hour.
 
Factors influencing the cost:

• Vehicle’s Year, Make, and Model: The type of ignition coil and its complexity varies, impacting the price of the part. 
• Type of Ignition Coil: Conventional coils are less expensive than coil-on-plug (COP) or distributorless ignition system (DIS) coils. 
• Parts Quality: Using a cheap, aftermarket part may be less expensive upfront but could fail sooner than an OEM (Original Equipment Manufacturer) part, according to this Reddit thread. 
• DIY vs. Professional Service: Doing the repair yourself can save on labor costs, but you’ll need the right tools and technical knowledge. 
• Location: Labor rates can vary depending on the shop and your geographic location. 

What to expect in a repair estimate:

• Parts: The ignition coil itself, which can range from $50 to over $600, depending on the model. 
• Labor: The cost for the mechanic’s time, typically charged by the hour, can vary widely. 
• Taxes and Fees: These are usually added to the final bill and are not included in the estimate. 

How to tell if a coil needs replacing a car?

You may need to replace a car’s ignition coil if you notice symptoms like the Check Engine Light coming on, the engine misfiring, rough idling, difficulty starting, stalling, slower acceleration, poorer fuel economy, or a backfiring sound from the exhaust. These issues indicate the coil isn’t providing sufficient voltage for proper spark plug ignition, disrupting the engine’s normal operation. 
Common Symptoms of a Failing Ignition Coil

• Check Engine Light: An illuminated Check Engine Light is a common indicator of a problem, often accompanied by a diagnostic code that can point to a misfiring cylinder. 
• Engine Misfires: You may feel a stuttering or jerking sensation, particularly during acceleration, as the coil fails to deliver a consistent spark to the spark plug. 
• Rough Idle: The engine may run unevenly or vibrate more than usual when idling. 
• Difficulty Starting: A bad coil can make it harder for the engine to start, or in some cases, prevent it from starting entirely. 
• Engine Stalling: The engine may cut out unexpectedly, sometimes during acceleration or at a stop. 
• Loss of Power: The car can feel sluggish and slow to respond to the accelerator pedal. 
• Poor Fuel Economy: With incomplete combustion from inconsistent sparks, the engine becomes less efficient, leading to increased fuel consumption. 
• Backfiring: Unburned fuel can enter the exhaust and ignite, causing a popping sound from the tailpipe. 
• Strong Fuel Smell: The presence of unburned fuel in the exhaust can also create a distinct fuel odor. 

What to Do Next
If you experience these symptoms, it’s a good idea to have the issue diagnosed. A mechanic can use an OBD-II scanner to pinpoint the exact cylinder with the misfire, helping to confirm a failing ignition coil.

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.