What happens when your ignition coil goes bad

A failing ignition coil typically causes engine misfires, rough idle, hesitation, poor fuel economy, hard starting or a no‑start, reduced power, and a Check Engine light (often flashing during active misfires). If ignored, it can overheat and damage the catalytic converter and oxygen sensors; on vehicles with a single coil, the engine may not start at all. Diagnosis usually involves reading fault codes, isolating the misfiring cylinder, and confirming with tests or by swapping coils; the fix is to replace the bad coil and often the spark plug or boot.

What the ignition coil does

An ignition coil transforms your car’s 12-volt battery power into the high voltage needed to create a spark at the spark plug—typically 20,000 to 40,000 volts or more. Many modern cars use coil‑on‑plug (one coil per cylinder), while others use coil packs or a single coil feeding multiple cylinders. Without a strong, properly timed spark, the air‑fuel mixture doesn’t burn efficiently or at all.

What you’ll notice when a coil fails

When a coil weakens or fails, the spark becomes inconsistent or disappears on one or more cylinders. That disruption shows up in drivability symptoms you can feel and see, especially under load or in wet weather.

• Misfire: shaking at idle, stumble under acceleration, or surging at steady speeds
• Loss of power and sluggish acceleration
• Hard starting, long cranks, or a complete no‑start (more likely with a single-coil system)
• Poor fuel economy and the smell of unburned fuel from the exhaust
• Check Engine light on; if it flashes, that indicates an active, catalyst‑damaging misfire
• Engine stalling, especially at stops or when transitioning off throttle
• Backfiring or popping from the intake/exhaust in severe cases
• Intermittent issues that worsen in rain/humidity or when the engine is hot

These symptoms often come and go as a marginal coil breaks down under heat and load, which is why a car may run fine cold but struggle once warmed up or during heavy acceleration.

Common reasons coils go bad

Ignition coils live in a harsh environment of heat, vibration, and high voltage. Several factors accelerate their wear or push them over the edge prematurely.

• Age and heat cycling: insulation breaks down over time, especially on engines with high under‑hood temperatures
• Worn or over‑gapped spark plugs that force the coil to work harder to jump the gap
• Oil intrusion into spark plug tubes from leaking valve cover gaskets
• Moisture and corrosion in coil connectors or boots
• Poor grounds, wiring damage, or loose coil connectors causing intermittent operation
• Aftermarket plugs or incorrect plug gap not matching spec
• Voltage spikes from charging system problems or jump‑start mishaps

Addressing the underlying stressors—like old spark plugs or oil in the plug wells—prevents repeat failures after you install a new coil.

Is it safe to keep driving?

If the Check Engine light is flashing or the engine is misfiring noticeably, minimize driving and fix the issue as soon as possible. Unburned fuel from a misfire can overheat and melt the catalytic converter and foul oxygen sensors, leading to much more expensive repairs. Some cars will temporarily disable the fuel injector on a misfiring cylinder to protect the catalyst, which reduces power further. If the car stalls or won’t accelerate safely, have it towed.

How to diagnose a bad ignition coil

Pinpointing a failing coil is straightforward on most modern vehicles with basic tools and a scan tool. The goal is to confirm the misfire, localize it to a cylinder, and verify that the coil—not the plug, injector, wiring, or engine mechanicals—is the cause.

1. Scan for codes and live data: look for P03xx misfire codes identifying a cylinder (e.g., P0302 for cylinder 2) and coil circuit codes.
2. Check misfire counters in live data to see which cylinder is acting up, especially under load.
3. Inspect the suspect coil and plug: look for cracks, carbon tracking, oil in the plug well, corrosion, or a torn boot.
4. Swap test: move the suspect coil to another cylinder and see if the misfire code follows it.
5. Test spark plugs: verify correct gap and condition; replace if worn or fouled.
6. Electrical checks: confirm coil power, ground, and control signal; check connector fit and wiring continuity.
7. Advanced testing: measure primary/secondary resistance (per service manual specs) or view coil waveform with an oscilloscope to assess dwell and spark line.
8. If the coil and wiring check out but the cylinder still misfires, test the injector, compression, and for vacuum/intake leaks.

A methodical approach prevents unnecessary parts swapping and ensures you don’t miss a wiring fault or a mechanical issue that mimics a coil problem.

Related fault codes you might see

Engine computers store diagnostic trouble codes (DTCs) that point toward misfires and coil circuits. These are the most common ignition‑related codes.

• P0300: Random/multiple cylinder misfire detected
• P0301–P0308 (and higher on larger engines): Misfire detected on a specific cylinder
• P0350–P0362: Ignition coil primary/secondary circuit malfunction (often identifies the affected coil)
• P1300–P1319 range on some makes (Toyota/Lexus, VW/Audi, etc.): Ignition system or coil faults

Use freeze‑frame data to see engine load, RPM, and temperature when the fault set; patterns here help reproduce the issue during testing.

Fixes, costs, and repair tips

Most coil problems are resolved by replacing the faulty coil and addressing any spark plug or wiring issues that contributed to the failure.

• Replace the bad coil (or the full set on high‑mileage engines to avoid repeat visits): $30–$150 per aftermarket coil; $80–$250 OEM per coil
• Spark plugs and boots (recommended with coil replacement if due): parts $40–$150; labor varies by access
• Labor time: typically 0.5–1.0 hour per coil; more if intake components must be removed
• Valve cover gasket(s) if oil is present in plug wells: $150–$500 parts and labor depending on engine
• If driven long with a misfire: catalytic converter $800–$2,500+; O2 sensors $100–$350 each
• Wiring repair for broken connectors or grounds: usually under $200 unless harness sections need replacement

Use coils and plugs that match the vehicle’s specifications, torque spark plugs correctly, and apply dielectric grease to boots where recommended to prevent future tracking and moisture intrusion.

Prevention and maintenance

Good ignition maintenance reduces stress on coils and helps them last longer, often well beyond 100,000 miles.

• Replace spark plugs at the interval and with the correct type and gap
• Keep oil and coolant out of plug tubes by fixing valve cover or coolant leaks promptly
• Inspect coil boots for cracks and carbon tracking; replace as needed
• Use dielectric grease on boots where specified to seal out moisture
• Ensure battery and charging system are healthy; voltage spikes can harm coils
• Avoid pressure‑washing directly over coils and connectors
• Address intake/vacuum leaks that lean out mixtures and increase ignition demand

Routine inspections during plug changes are the best time to catch early signs like cracked boots or light carbon tracking before they cause a misfire.

When it isn’t the coil

Other faults can mimic a bad coil: a failing spark plug, a clogged or leaking fuel injector, low compression (worn rings, valves), vacuum leaks near one cylinder, a faulty crank or cam sensor, or a PCM driver or wiring issue feeding the coil. If multiple cylinders share a coil pack or bank and all misfire together, consider shared power/ground issues or a failed coil pack rather than a single coil-on-plug.

Summary

A bad ignition coil disrupts spark, causing misfires, rough running, power loss, and a Check Engine light; prolonged driving risks costly catalytic converter damage. Confirm the fault with scans, inspection, and a swap test, then replace the coil and any worn plugs or boots. With proper maintenance—fresh plugs, sealed plug wells, and sound wiring—coils typically deliver long, trouble‑free service.

Will a car run with a bad ignition coil?

A car might still run with a bad ignition coil, but it will likely run roughly and could cause further damage, especially if it has a multi-coil system. If your car uses a single ignition coil and it fails, the engine won’t get a spark to ignite the fuel, and it won’t run at all. Driving with a bad coil can lead to engine misfires, damage the catalytic converter, and eventually cause severe engine damage.
 
What Happens with a Bad Ignition Coil

• Engine Misfires: A faulty coil prevents the spark plug from firing, causing a misfire in one or more cylinders. 
• Difficulty Starting: You might notice a sputtering or jerky engine start, or the engine may struggle to start altogether. 
• Rough Running: The engine will run unevenly and with less power because not all cylinders are contributing to the engine’s operation. 

Why You Shouldn’t Drive with a Bad Coil 

• Catalytic Converter Damage: Opens in new tabUnburnt fuel from misfiring cylinders can enter the exhaust and ignite in the hot catalytic converter, leading to extreme temperatures that damage the converter.
• Oil Dilution: Opens in new tabFuel can seep past the piston rings, diluting the engine oil and potentially causing bearing damage to the crankshaft and connecting rods.
• Engine Stress: Opens in new tabA misfiring cylinder can create stress on the engine, potentially leading to crankshaft and bearing failure over time.

This video explains the symptoms of a bad ignition coil: 1mEasyAutoFixYouTube · Mar 25, 2023
What to Do if You Suspect a Bad Coil

• Get it Tested: It’s best to have the ignition coils tested to determine which one is faulty. 
• Replace Immediately: To prevent further damage to your vehicle, get the faulty coil replaced as soon as possible. 

How to tell if a coil needs replacing?

To determine if an automotive ignition coil needs replacement, look for symptoms like a rough idle, engine misfires, poor fuel economy, a Check Engine light, or a stalling/sputtering engine. For a vape coil, signs include a burnt taste, a bubbling noise, weak vapor production, a leaking tank, or a diminished e-liquid flavor. 
For an automotive ignition coil:

• Misfires and rough idle: The engine may shake, stumble, or feel jerky, especially during acceleration. 
• Stalling or poor starting: The engine may stall or crank for a long time before starting. 
• Reduced power: You might notice a decrease in the vehicle’s power, making acceleration sluggish. 
• Poor fuel economy: Inefficient combustion due to weak spark can lead to lower miles per gallon. 
• Check Engine light: A fault in the ignition system often triggers the Check Engine light. 
• Backfiring: Unburned fuel may exit the exhaust and ignite, causing a loud popping or backfiring sound. 

For a vape coil:

• Burnt taste: A common sign of an aging coil is an unpleasant, burnt taste when inhaling. 
• Weak vapor or flavor: A diminished flavor and reduced production of vapor can indicate a worn-out coil. 
• Tank leaking or gurgling: A faulty coil can lead to e-liquid leaks or a hollow gurgling sound from the device. 
• Harsh hits: The device may produce harsher, less pleasant hits. 
• Bubbling sounds: A hollow bubbling noise while using the device can also signal a coil needs replacing. 

How can you tell if an ignition coil is bad?

You can tell an ignition coil is bad by a variety of symptoms, including a lit Check Engine Light, engine misfires, rough idling, lack of power, poor acceleration, a strong smell of gas, and potentially even black smoke or backfiring from the exhaust. A common diagnostic method involves using an OBD-II scanner to check for misfire codes, or physically unplugging each coil (on newer vehicles) to see if the engine’s performance changes or doesn’t change.
 
This video explains how to identify a bad ignition coil: 53sHonest Mechanic ColoradoYouTube · Jan 11, 2024
Common Symptoms of a Bad Ignition Coil

• Check Engine Light: A flashing or steady Check Engine Light is a strong indicator of an ignition system problem, and a misfire code on an OBD-II scanner can pinpoint an ignition coil issue. 
• Engine Misfire: You might feel your engine sputtering, shaking, or jerking, particularly when idling or accelerating. This happens because a failing coil isn’t providing a consistent spark to the combustion chamber. 
• Rough Idle and Shaking: The engine’s vibrations might become more noticeable at idle, leading to a rough-running feel that can be felt through the steering wheel and the entire vehicle. 
• Reduced Power and Hesitation: A malfunctioning coil can lead to a loss of engine power, making your car feel sluggish and less responsive when accelerating. 
• Poor Fuel Economy: With inefficient fuel combustion, your engine has to work harder, consuming more gas and resulting in fewer miles per gallon. 
• Smell of Gasoline: Unburned fuel can escape the combustion cylinder and exit through the exhaust system, leading to a noticeable smell of gas. 
• Black Smoke or Backfiring: The excess unburned fuel can ignite in the exhaust system, producing thick black smoke or causing a loud, cracking sound known as backfiring. 
• Hard Starting: Your car might take several attempts to start, or the engine could feel jerky during the initial startup process. 

How to Diagnose (Basic Steps)

• Use an OBD-II Scanner: Opens in new tabPlug the scanner into your vehicle’s port to read fault codes, which can identify specific cylinders that are misfiring. 
• Perform a “No-Change” Test (for coil-on-plug systems): Opens in new tabWith the engine running, carefully disconnect the electrical connector from each ignition coil one at a time. If the engine’s performance doesn’t change when you disconnect a particular coil, that coil is likely bad. 

This video demonstrates how to test for a bad ignition coil: 1mBuddy’s DIYYouTube · Jun 25, 2020
If you’re unsure about these steps or aren’t comfortable with car diagnostics, it’s best to take your vehicle to a qualified mechanic for professional assessment and repair.

What does a bad coil feel like?

It’s Difficult to Drive Without Misfiring. If you can no longer drive without your engine misfiring, then you might have a faulty coil. When your car misfires, it vibrates or shakes while idling, and while you’re driving, it feels jerky or sputtery.