What Happens When an Engine Coil Fails?

When an engine ignition coil fails, the affected cylinder loses spark or gets a weak spark, causing a misfire that leads to rough running, reduced power, poor fuel economy, and a check-engine light (often flashing under load). Continued driving can overheat and damage the catalytic converter as unburned fuel enters the exhaust. In practical terms, you’ll likely feel shaking at idle or during acceleration, experience sluggish performance, and may need to stop driving to prevent costly exhaust damage.

How Ignition Coils Work and Why They Fail

Ignition coils transform the car’s 12-volt battery power into tens of thousands of volts needed to jump the spark plug gap and ignite the air-fuel mixture. Most modern vehicles use coil-on-plug (COP) units—one coil per cylinder—while others use small multi-tower coil packs. Coils can fail from internal short circuits, open windings, insulation breakdown (arcing), heat and vibration, oil or coolant contamination in the plug wells, moisture intrusion, or being overworked by worn or incorrectly gapped spark plugs.

Symptoms You’ll Notice

Drivers typically notice a misfire that varies with load and temperature. The engine control module (ECM) may illuminate a check-engine light (CEL)—steady for stored faults or flashing for an active, catalyst-damaging misfire. Below are common signs tied to coil failure.

• Shaking or rough idle; the engine feels uneven and may vibrate noticeably.
• Hesitation or stumbling on acceleration; power loss, especially under heavy throttle or hills.
• Check-engine light on or flashing; trouble codes such as P0300 (random misfire), P0301–P0308 (cylinder-specific misfire), and P0351–P0358 (ignition coil primary/secondary circuit faults). Some makes use manufacturer-specific coil codes.
• Poor fuel economy due to incomplete combustion and the ECM adding fuel in response to oxygen sensor readings.
• Fuel smell, popping, or backfire from unburned fuel in the exhaust; potential sulfur/rotten-egg smell from an overheated catalytic converter.
• Hard starting or stall at idle, especially when the coil fails completely.
• Intermittent issues that worsen as the engine warms—thermal failures are common.

While symptoms overlap with other faults (fuel injector, vacuum leak, compression loss), a failed coil is among the most common causes of single-cylinder misfires in modern vehicles.

What’s Happening Inside the Engine

With a weak or absent spark, the affected cylinder doesn’t burn its mixture reliably. Oxygen sensors detect extra oxygen from the misfire, prompting the ECM to adjust fueling. Raw fuel can enter the catalytic converter, raising its temperature beyond safe limits; a flashing CEL warns of this condition. Repeated misfires can melt converter substrates and foul oxygen sensors. Depending on the failure type—primary circuit, secondary insulation, or boot tracking—the misfire may be constant or triggered by heat, humidity, or high load.

Diagnostic Steps

A structured diagnosis avoids parts-chasing. Basic tools include an OBD-II scanner, visual inspection, and, ideally, a spark tester or oscilloscope. Here’s a typical workflow technicians use.

1. Scan for codes and note freeze-frame data (engine load, RPM, temperature) to see when the misfire occurs.
2. Identify the cylinder(s) misfiring (e.g., P0302 = cylinder 2) and any coil circuit codes (e.g., P0352).
3. Perform a “swap test”: move the suspected coil to another cylinder and see if the misfire code follows. If it does, the coil is likely faulty.
4. Inspect spark plugs and coil boots for wear, cracks, carbon tracking, oil or coolant in plug wells, and water intrusion.
5. Check plug condition and gap; severely worn plugs increase coil load and can precipitate coil failure.
6. Verify coil power, ground, and ECM trigger with a test light or scope; confirm no wiring/connectors are damaged or corroded.
7. If needed, measure coil resistance (primary/secondary) or analyze ignition waveforms; compare to specifications.
8. Rule out non-ignition causes: injector operation, fuel pressure, vacuum leaks (multiple-cylinder misfires), and compression on the affected cylinder.
9. Check for technical service bulletins (TSBs) and PCM/ECM software updates addressing misfire or coil issues.
10. Road test under the conditions found in freeze-frame data to confirm the fix.

This process isolates whether the coil itself, its control circuit, the plug/boot, or another engine fault is to blame, reducing unnecessary parts replacement.

Can You Keep Driving?

If the check-engine light is flashing or the vehicle runs very rough, minimize driving and avoid high load; continued operation risks catalytic converter damage that can cost far more than coils and plugs. For a mild, steady misfire with a solid CEL, you may be able to drive a short distance to a shop, but it’s safer to address it promptly. If the engine stalls, smells strongly of fuel, or the exhaust gets extremely hot, stop and arrange a tow.

Common Causes and Prevention

Coils are robust but vulnerable to heat, contamination, and electrical stress. Addressing upstream issues extends their life. Consider the following contributors and preventative steps.

• Overdue or incorrectly gapped spark plugs increase coil workload and heat—replace plugs on time with the specified gap and type.
• Oil in spark plug tubes from valve cover gasket leaks degrades boots and insulators—repair leaks and replace affected boots.
• Coolant or water intrusion (car washes, leaking cowl seals) promotes arcing—keep wells dry and avoid pressure-washing the engine.
• Poor-quality aftermarket coils may fail early—use OEM or reputable brands, especially on turbo/high-compression engines.
• Wiring/connector corrosion or weak grounds cause intermittent coil operation—clean and repair as needed.
• Battery/charging issues (low voltage, voltage spikes) stress ignition components—maintain a healthy battery and alternator.
• Repeated failures on one cylinder can indicate carbon tracking on the plug insulator or a lean condition—inspect and correct root causes.

By controlling heat, contamination, and electrical load, you reduce the likelihood of repeat coil failures and misfires.

Repair Options and Cost

On coil-on-plug engines, it’s common to replace the failed coil and the spark plug on that cylinder; many shops recommend replacing all plugs if they’re due by mileage. Boots are inexpensive and often replaced with the plug. Some owners replace all coils preventively on high-mileage vehicles, but it’s not mandatory if others test fine.

Typical costs (vary by vehicle and region, 2025):

• Ignition coil (mainstream vehicles): $40–$150 each; premium/Euro brands: $120–$300+ each.
• Spark plugs: $8–$30 each (iridium/platinum), plus labor.
• Labor: 0.3–1.0 hour per coil on accessible engines; up to 2–3 hours if intake manifolds must be removed.
• Diagnostic fee: $100–$180 is typical for professional testing.
• Catalytic converter (if damaged by prolonged misfire): $800–$2,500+ per unit, more on some V6/V8 or Euro models.

Using quality parts, installing new plugs/boots, applying dielectric grease on boots, and torquing plugs correctly help ensure a lasting repair.

Special Cases

Vehicles with coil packs (wasted-spark systems) can lose spark on two cylinders when one coil segment fails. Turbocharged and direct-injection engines tend to expose weak coils under boost, so misfires may only appear at high load. Hybrids that frequently start/stop the engine can mask intermittent faults until warm. Some makes have TSBs addressing moisture ingress or updated coil designs; checking for these can save time and money.

Quick Tips for Owners

A few practical steps can help you respond effectively to coil issues and prevent recurrence.

• Do not ignore a flashing check-engine light; reduce load and get the car checked promptly.
• Replace spark plugs at the interval in your owner’s manual; verify correct plug type and gap.
• Inspect for valve cover leaks and fix them before they contaminate plug wells.
• Avoid pressure-washing the engine bay; keep coil areas dry and clean.
• Use OEM or high-quality aftermarket coils; cheap units often fail early.
• If one coil fails and plugs are old, replace the plug and boot on that cylinder at minimum.
• If the same cylinder repeats failures, check wiring, grounds, plug gap, and for carbon tracking or lean conditions.

These practices reduce misfire risks and protect expensive exhaust components.

Summary

A failed ignition coil disrupts spark, causing a misfire that triggers rough running, reduced power, and a check-engine light. Left unchecked, it can overheat and damage the catalytic converter. Diagnosis typically involves scanning for codes, inspection, and swap testing. Fixes range from replacing a single coil and plug to addressing leaks, wiring, or software updates. Prompt attention, quality parts, and on-time spark plug service minimize the chance of recurrence and help keep repair costs in check.