What Happens When an Engine Coil Goes Bad

When an engine (ignition) coil goes bad, the affected cylinder misfires, leading to rough idle, loss of power, poor fuel economy, and a steady or flashing Check Engine light; continued driving can damage the catalytic converter. Diagnosis typically shows misfire and coil-circuit codes, and the remedy is to replace the failed coil—often along with the spark plug and coil boot.

How an Ignition Coil Fits Into Modern Engines

An ignition coil transforms the car’s 12-volt battery power into tens of thousands of volts to jump the spark plug gap and ignite the air-fuel mixture. Most late-model vehicles use coil-on-plug (COP) systems with one coil per cylinder, while some use a coil pack or a single coil with wires to multiple plugs. When a coil fails, the cylinder loses or weakens spark, and combustion becomes erratic or stops entirely.

What You’ll Notice Behind the Wheel

The most immediate clues come from how the car feels and sounds during everyday driving. The following points outline common real-world symptoms owners report when a coil fails.

• Rough idle, shaking, or vibration—especially at stoplights
• Noticeable loss of power, sluggish acceleration, or hesitation
• Increased fuel consumption and fuel smell from the exhaust
• Check Engine light on; flashing light under load indicates an active misfire
• Hard starts or stalling, particularly when the engine is hot or in wet weather
• Popping from the exhaust or intake during severe misfire events

These symptoms often worsen under load—like climbing hills or merging—because the ignition system must work harder to fire under higher cylinder pressure.

What Your Dashboard and Scanner Will Show

On modern OBD-II vehicles, a failing coil leaves data breadcrumbs. A basic scan tool or professional diagnostic scanner can reveal codes and live data that pinpoint the problem.

• P0300: Random/multiple cylinder misfire
• P0301–P0308: Cylinder-specific misfire (number indicates the cylinder)
• P0351–P0358: Ignition coil primary/secondary circuit faults (coil control circuit issues)
• Fuel trims skewed (often positive) as the engine management tries to compensate
• Misfire counters in Mode $06 or enhanced data showing which cylinder drops out

A flashing Check Engine light signals a catalyst-damaging misfire—reduce load and address it promptly to avoid expensive downstream damage.

Why It Fails: Common Root Causes

Coils are robust but live in a harsh environment. The following factors commonly drive premature failure.

• Heat and vibration degrading internal windings and insulation
• Oil intrusion from leaking valve cover gaskets soaking coil boots
• Moisture ingress or corrosion in coil connectors
• Excessive spark plug gap or worn plugs overworking the coil
• Carbon tracking on plugs or boots providing an easier path to ground
• Aftermarket low-quality coils or mismatched components
• Wiring or ECM driver faults causing coil control issues

Addressing the underlying cause—such as oil leaks or overdue spark plugs—prevents repeat failures and protects new coils.

Risks of Continuing to Drive

Driving for extended periods with a misfire from a bad coil carries real mechanical and financial risks.

• Catalytic converter overheating and meltdown from unburned fuel (costly replacement)
• Fuel washing down cylinder walls, thinning engine oil and accelerating wear
• Fouled spark plugs and oxygen sensors
• Potential misfire-related damage to turbochargers on boosted engines

Short, gentle trips to a repair facility are generally acceptable, but sustained driving under load can quickly escalate repair costs.

How to Confirm a Bad Coil

Basic testing can be decisive, though best practices depend on your vehicle’s coil design and tools available.

• Swap test: Move the suspected coil to another cylinder and see if the misfire code follows
• Visual inspection: Look for oil, moisture, cracked housings, burned boots, or carbon tracks
• Spark plug check: Inspect and measure gap; replace worn or fouled plugs
• Power/ground/signal verification: Ensure coil has proper voltage, ground, and control signal
• Scope testing (advanced): Analyze coil control and secondary patterns for weak or no spark
• Resistance checks: Sometimes indicative, but many modern coils don’t reliably test with an ohmmeter

Combining a swap test with a fresh spark plug often isolates the fault quickly and avoids misdiagnosis of injectors or sensors.

Repair Options and Typical Costs

Most coil failures are resolved with straightforward parts replacement, but parts quality and related maintenance matter.

• Coil replacement: About $30–$120 per coil for quality parts; labor 0.3–1.2 hours per cylinder depending on access
• Spark plugs and boots: $8–$25 per plug; boots $5–$20 each; labor often combined with coil service
• Full set replacement: Common on high-mileage vehicles to avoid serial failures; parts can run $200–$900+
• Catalytic converter (if damaged): $900–$2,500+ parts and labor, depending on vehicle

Use OEM or reputable aftermarket coils, torque plugs correctly, and apply dielectric grease to boots where specified. Replacing overdue plugs alongside the coil reduces repeat misfires.

Can You Drive With a Bad Coil?

It’s generally safe only for a brief, low-load trip to a shop. If the Check Engine light flashes, ease off the throttle immediately. Some technicians may disable fuel to the dead cylinder on certain engines to protect the catalyst, but that’s not a DIY move on modern vehicles and may not be supported by all engine management systems.

Prevention and Maintenance

Preventive care reduces the odds of coil failures and misfires, especially on high-mileage engines.

• Replace spark plugs at the factory interval and verify correct gap and heat range
• Fix valve cover gasket leaks promptly to keep oil out of plug wells
• Avoid pressure-washing the engine; keep connectors dry and secure
• Use dielectric grease on boots where recommended and ensure boots are seated fully
• Address misfires early to protect the catalytic converter and oxygen sensors

Staying current on basic ignition maintenance is the simplest way to extend coil life and keep the engine running smoothly.

Bottom Line

A bad ignition coil causes a misfire that hurts performance, economy, and emissions—and can quickly snowball into catalytic converter damage if ignored. Scan for codes, verify with a swap test or inspection, and replace the failed coil along with any worn plugs or boots. Prompt, proper repair is far cheaper than waiting for collateral damage.

Summary

When an engine coil fails, you’ll feel rough running, see a Check Engine light, and likely log misfire/coil codes. The fix is typically a new coil and fresh plug or boot, plus correcting underlying issues like oil leaks or worn plugs. Don’t drive hard with an active misfire—protect your catalytic converter and engine by diagnosing and repairing the issue quickly.