What Car Coils Do—and Why They Matter

In most automotive conversations, “the coils” refers to ignition coils: they transform a car’s 12-volt battery power into tens of thousands of volts to fire the spark plugs, igniting the air–fuel mixture in gasoline engines. Without functioning ignition coils, a petrol engine will misfire or fail to run. The term can also describe other coils in a car—such as suspension coil springs or air-conditioning coils—but ignition coils are the critical components that make combustion possible.

How an Ignition Coil Works

An ignition coil is a compact step-up transformer. It has a primary winding (few turns of thicker wire) and a secondary winding (many turns of fine wire) wrapped around an iron core. When the engine control unit (ECU) energizes the primary winding, it builds a magnetic field. Rapidly switching the current off collapses that field, inducing a high-voltage pulse—often 20,000 to 40,000 volts, and sometimes higher—in the secondary winding. That pulse jumps the gap at the spark plug, igniting the mixture. The ECU times this precisely (called “dwell” and spark timing) to match load and RPM, ensuring strong, clean combustion.

Common ignition coil architectures

Modern engines use several coil layouts. The following list explains the major types you may encounter and how they route spark energy.

• Single coil with distributor: One coil feeds a distributor that mechanically routes spark to each cylinder; common in older vehicles.
• Coil pack (wasted-spark): Multiple coils each fire two cylinders, sparking one on compression and one on exhaust (“wasted”) stroke.
• Coil-on-plug (COP): One coil per spark plug, mounted directly on the plug for minimal energy loss and precise control; most common on late-model engines.

While each architecture achieves the same end—creating a reliable spark—coil-on-plug systems typically deliver the most efficient, consistent ignition with fewer high-voltage losses and better diagnostics.

Where Coils Fit in Different Engines

Gasoline (spark-ignition) engines rely on ignition coils. Hybrids with gasoline engines use them too. Pure electric vehicles do not have ignition coils because there’s no combustion. Diesel engines use compression ignition and do not need spark plugs or ignition coils; they may use glow plugs for cold starts, which are electrically heated but not the same as ignition coils.

Common Symptoms of a Failing Ignition Coil

Failing coils often announce themselves through drivability problems. Watch for the following issues, especially under load or during damp weather.

• Intermittent or persistent misfire (rough idle, hesitation, shudder under acceleration)
• Hard starting or no start in severe cases
• Reduced power and poor fuel economy
• Check Engine Light (often flashing during active misfire)
• Fuel smell from the exhaust, occasional backfires, or elevated emissions
• Catalytic converter overheating or damage if misfires are ignored

These symptoms can overlap with bad spark plugs, vacuum leaks, fueling issues, or compression problems, so proper diagnosis is important before replacing parts.

Why Ignition Coils Fail

Coils are rugged but exposed to heat, vibration, and high voltage. The issues below are among the most common failure drivers.

• Heat and vibration causing internal insulation breakdown or cracked housings
• Oil intrusion from leaking valve cover gaskets soaking coil boots and windings
• Moisture and corrosion in connectors or on plug wells
• Excessive spark plug gap or worn plugs forcing the coil to work harder
• Poor engine grounds or voltage spikes stressing the coil and drivers
• Low-quality aftermarket parts with inferior insulation or windings

Addressing underlying causes—like fixing gasket leaks and replacing worn plugs—helps prevent repeat coil failures and restores ignition reliability.

How to Diagnose Coil Problems

Systematic checks can confirm whether a coil is at fault or if another issue is mimicking coil failure.

• Scan tool: Look for P0300 (random) or P0301–P0308 (cylinder-specific) misfires and P0351–P0358 coil circuit faults; review freeze-frame and Mode 6 data.
• Swap test: Move a suspect coil to another cylinder to see if the misfire follows.
• Inspect spark plugs: Check gap, wear, fouling, and cracks; a bad plug can make a good coil look bad.
• Visual check: Look for oil in plug tubes, carbon tracking on insulators, cracked boots, or corroded connectors.
• Electrical tests: Measure primary/secondary resistance and perform a coil current ramp or secondary waveform check with an oscilloscope.
• Power/ground verification: Confirm coil power supply and ECU coil-driver command with a test light or scope.

Combining scan data with a swap test and basic inspections often isolates the fault quickly, saving time and unnecessary parts replacement.

Replacement, Cost, and Maintenance

Ignition coil service is straightforward on most engines, especially coil-on-plug setups. Consider the best practices below when repairing or maintaining the system.

• Typical part cost ranges from about $30 to $150 per coil; labor is often 0.3–1.0 hours per accessible coil.
• Replace spark plugs at the same time if they’re due; excessive gaps strain coils.
• Renew coil boots where applicable and apply dielectric grease inside the boot to prevent tracking.
• Torque spark plugs to spec and avoid overtightening coils; ensure clean, dry plug wells.
• Use quality OEM or reputable aftermarket coils to ensure proper insulation and longevity.
• Avoid prolonged driving with a misfire; unburned fuel can destroy the catalytic converter.
• Replace one versus all: It’s acceptable to replace a single failed coil, but high-mileage sets may benefit from replacing all to prevent repeat visits.

Attentive ignition maintenance—especially timely spark plug service and fixing oil leaks—reduces coil stress and extends component life.

Safety Notes

Ignition systems generate dangerous voltages. Keep the following precautions in mind during any inspection or repair.

• Never touch coils or plug leads with the engine running; secondary voltage can exceed 40,000 volts.
• Do not pull plug wires or COP coils off a running engine to “check spark”; this can shock you and damage electronics.
• Be cautious if you have a pacemaker or implant; avoid exposure to ignition high-voltage pulses.
• If disconnecting the battery, follow manufacturer procedures to preserve module memory and settings.

Respecting the ignition system’s high voltage and using proper tools and procedures minimizes risk to you and the vehicle’s electronics.

Beyond Ignition Coils: Other Coils in a Car

The word “coil” can describe multiple components, which can be confusing. Here’s how other automotive coils differ from ignition coils.

• Suspension coil springs: Mechanical steel springs supporting vehicle weight and ride height.
• Air-conditioning coils: Evaporator and condenser “coils” that exchange heat in the HVAC system.
• Solenoids and relays: Electromagnetic coils used to actuate valves or switch circuits (e.g., transmission solenoids, starter relay).

While these parts share the term, only ignition coils create the high-voltage spark required for combustion in gasoline engines.

Summary

Ignition coils convert 12-volt battery power into high-voltage pulses to fire spark plugs—central to every gasoline engine’s operation. Modern coil-on-plug systems deliver efficient, precisely timed sparks, but heat, oil leaks, worn plugs, and electrical issues can cause failures. Recognizing symptoms, diagnosing methodically, and pairing coil replacement with proper spark plug service will keep engines running smoothly and protect expensive components like catalytic converters.