How to Tell If Your Cylinder’s Ignition Coil Is Bad

A bad ignition (cylinder) coil typically causes a noticeable misfire—rough idle, loss of power under load, harder starts, worse fuel economy, and a Check Engine light with codes like P0301–P0308 (misfire by cylinder) or P0351–P0358 (coil circuit). To confirm, swap the suspect coil to another cylinder and see if the misfire follows, check for strong spark with a spark tester, and verify the coil has power, ground, and a trigger signal. If the misfire follows the coil or the coil fails these checks, the coil is bad and should be replaced.

What the “Cylinder Coil” Is and Why It Fails

Most modern cars use coil-on-plug (COP) units—one coil per cylinder—while some use a shared “coil pack” that feeds multiple cylinders via plug wires. Coils convert the 12-volt supply into tens of thousands of volts needed to fire the spark plug. Heat, vibration, oil or coolant contamination, and worn spark plugs can overwork and eventually damage a coil.

Key Symptoms of a Bad Ignition Coil

The following signs commonly indicate a failing or failed ignition coil and can help you differentiate from other engine issues.

• Rough idle, shaking, or stalling—especially at stops or when the engine is warm.
• Hesitation or bucking under acceleration or uphill loads.
• Poor fuel economy and reduced power; sometimes a fuel smell from the tailpipe.
• Check Engine light with misfire codes (P0300 random, P0301–P0308 specific cylinder) or coil circuit codes (P0351–P0358).
• Difficulty starting, especially in damp or cold conditions.
• Audible “snap” or visible arcing around the coil or boot in the dark (insulation breakdown).
• Exhaust sulfur/rotten-egg smell or glowing/hot catalytic converter from raw fuel accumulation.

While one symptom alone isn’t conclusive, a cluster of these signs—especially coupled with matching OBD-II codes—strongly suggests coil trouble.

Quick Checks You Can Do

Safety First

Ignition systems produce high voltage that can shock you. Work with the engine off when unplugging coils, keep hands and tools clear of moving parts, and use an inline spark tester rather than pulling a plug wire to “watch for spark.” Disconnect the battery before meter testing connectors.

Step-by-Step Checks Without Special Tools

These basic checks can isolate a bad coil on most vehicles in minutes.

1. Scan for codes: Note any P030X (cylinder-specific misfire) or P035X (coil circuit) codes and which cylinder they reference.
2. Swap test: Move the suspected coil to another cylinder (e.g., Cylinder 2 to Cylinder 4). Clear codes, drive, and rescan. If the misfire code moves with the coil, the coil is faulty.
3. Inspect the spark plug and boot: Look for oil in the plug well, carbon tracking (thin black lightning-like lines), cracked boots, or worn electrodes. Replace damaged plugs/boots and recheck.
4. Check the connector: Ensure the coil’s electrical connector is fully seated, pins are clean and not corroded, and the wire loom isn’t brittle or broken.
5. Power/ground check: With key ON (engine off), verify the coil has battery voltage on the power feed using a multimeter or test light. A blown fuse or relay can mimic a bad coil.
6. Humidity test: Lightly mist water around the coil/boot area in the dark. If misfire worsens or sparking is visible, insulation is compromised.

If the misfire follows the coil or improves when you replace a suspect boot or plug, you’ve identified the likely fault. If not, proceed to more detailed tests.

Using a Basic OBD-II Scanner

An inexpensive scanner can provide decisive clues about coil health.

• Codes to watch: P0301–P0308 (misfire by cylinder), P0300 (random), P0351–P0358 (coil primary/secondary circuit).
• Freeze-frame data: Note RPM, load, and engine temp when the code set; load-related misfires often implicate coils/plugs.
• Misfire counters (live data on some cars): Rising counts on one cylinder at idle or under load point to that cylinder’s coil/plug/injector.

Persistent cylinder-specific misfires that mirror coil swaps are strong evidence of a defective coil rather than a broader fuel or compression issue.

Deeper Diagnostics (If You Have Tools)

These tests provide further confirmation but may require specifications from your service manual and basic tools.

Multimeter Resistance Tests

Measuring coil resistance can catch some failures, though many modern COP coils pass resistance tests yet fail under heat/load.

• Primary resistance (low side): Often around 0.3–2.0 ohms between the coil’s power and control pins. Values outside spec suggest internal damage.
• Secondary resistance (high side): Older designs may read roughly 6,000–15,000 ohms from coil output to ground; many COP coils hide the secondary and aren’t reliably testable this way.
• Compare cylinders: If specs aren’t available, compare readings across coils—an outlier is suspect.

Always use the exact specs for your vehicle; acceptable ranges vary widely by design, and temperature affects readings.

Spark Tester, Noid/Test Light, and Oscilloscope

These tools help separate a bad coil from wiring, PCM, or spark plug faults.

• Inline adjustable spark tester: Crank the engine and watch for a strong, blue spark. Weak/no spark on one coil but good on others indicates a bad coil or its feed/ground.
• Test light/noid on trigger wire: Confirms the PCM is pulsing the coil. If there’s a pulse and good power/ground but no spark, the coil is bad.
• Lab scope (advanced): Check coil primary voltage and current ramp; irregular dwell/turn-off or flat current ramp points to internal coil fault or driver issues.

If power, ground, and trigger are good but spark is weak or absent, replace the coil. If trigger is missing, investigate wiring or PCM drivers instead.

Common Causes of Coil Failure

Understanding root causes prevents repeat failures and can reveal related issues that need attention.

• Worn or wrong-gap spark plugs increasing firing voltage demand.
• Oil or coolant intrusion into plug wells from valve cover or head gasket leaks.
• Heat soak from turbochargers or cramped engine bays; deteriorated coil boots/insulation.
• Water ingress, especially after engine washing or heavy rain.
• Poor electrical grounds, corroded connectors, or chafed harnesses causing overcurrent or intermittent operation.
• Aftermarket coils of poor quality or mismatched part numbers.

Addressing these contributors—especially plugs and leaks—helps your replacement coil last.

Distinguishing Coil Problems from Spark Plug or Injector Issues

Because coils, plugs, and injectors all influence combustion, it’s useful to tell them apart before replacing parts.

• Coil vs plug: If the misfire follows the coil during a swap, it’s the coil; if it follows the plug, the plug is the issue. Carbon tracking on the insulator points to plug/boot-related arcing.
• Injector clues: A dead injector typically causes a cold exhaust runner on that cylinder, lean codes, or no change when unplugging the injector at idle. Fuel trim data may skew lean on that bank.
• Compression/mechanical: Low compression yields a steady miss that won’t follow coil/plug swaps and shows low readings on a compression test.

Systematic swaps and basic measurements can narrow the fault to the right component quickly and cheaply.

Can You Drive with a Bad Coil?

It’s not recommended. Driving with a misfire can overheat and damage the catalytic converter, foul the spark plug, and increase fuel consumption. A flashing Check Engine light indicates active catalyst-damaging misfire—reduce load and have the vehicle serviced immediately. If you must move the car, drive gently for the shortest distance.

Repair Options and Typical Costs (2025)

Costs vary by vehicle and access, but these ranges are common in North America.

• Coil-on-plug (each): $40–$180 for OEM or quality aftermarket; labor $50–$150 per coil depending on access.
• Coil packs (shared): $80–$400 parts; labor $80–$200.
• Spark plugs: $8–$25 each (iridium/platinum); consider replacing if overdue or if a coil failed.
• Coil boots: $5–$20 each; inexpensive and worth replacing with the coil.
• Connector/harness repair: $20–$100 in parts; labor varies with routing and access.

Replace only the failed coil unless the vehicle has high mileage, a known batch issue, or multiple coils are failing. Always fix oil leaks and install fresh plugs to prevent repeat failures.

Preventing Future Coil Failures

Simple maintenance choices can significantly extend coil life and reduce misfires.

• Replace spark plugs at the recommended interval and set correct gap if required.
• Fix valve cover gasket leaks and keep plug wells dry and clean.
• Use quality, application-correct coils; avoid cheap no-name parts.
• Ensure battery and charging system are healthy; low voltage stresses ignition components.
• Avoid pressure-washing coil areas; if they get wet, dry thoroughly before driving.

These steps lower ignition voltage demand and protect the coil’s insulation from heat and contamination.

Summary

Suspect a bad ignition coil when you have a cylinder-specific misfire, power loss under load, and matching OBD-II codes. Confirm by swapping coils, checking for strong spark, and verifying power/ground/trigger. Replace the coil if the misfire follows it or it fails basic tests, and address underlying causes such as worn plugs or oil leaks. Prompt repair prevents catalytic converter damage and restores smooth performance.

What are the symptoms of a bad coil?

Symptoms of a bad ignition coil include a blinking or solid check engine light, rough idling, engine misfires (causing sputtering or shaking), stalling, poor acceleration and loss of power, unusual engine noises (like backfiring), and a gasoline smell from the exhaust. If you notice these symptoms, it’s best to have a mechanic diagnose the issue, as it can lead to further engine damage and poor fuel economy.
 
Engine Performance Issues

• Engine Misfires: You might feel or hear a sputter, cough, or jerk, especially noticeable when idling or accelerating. 
• Rough Idling: The engine may vibrate, judder, or feel unsteady when the car is stopped or in neutral. 
• Loss of Power: The engine may feel sluggish, and you’ll have trouble gaining speed. 
• Stalling: The engine could stall or die out unexpectedly, particularly when you’re stopped or slowing down. 
• Hard Starts: It may take longer than usual to start the engine. 

Unusual Noises and Smells 

• Banging Noises: The engine might make loud, banging sounds. 
• Gasoline Smell: You may smell fuel, especially from the exhaust, due to unburned fuel entering the exhaust system. 
• Black Smoke: In some cases, black smoke may be seen coming from the tailpipe. 

Warning Lights and System Malfunctions

• Check Engine Light: Opens in new tabA blinking check engine light usually indicates a serious engine problem, such as a misfire from a faulty coil. 
• Blinking Check Engine Light: Opens in new tabA blinking light is a critical warning that suggests an issue is happening and should be addressed immediately to prevent severe damage. 

What to Do
If you notice any of these symptoms, it is recommended to schedule an appointment with a qualified mechanic. They can diagnose the problem with an OBD2 scanner to check for misfire codes and confirm if a faulty ignition coil is the cause. Ignoring these signs can lead to decreased fuel efficiency and potential, more costly engine damage.

How do I test if my ignition coil is bad?

To check a bad ignition coil, look for signs like a blinking Check Engine Light, engine misfires, poor fuel economy, and decreased power. For a more definitive test, use a multimeter to measure the coil’s resistance or perform a spark test by connecting a spark tester to a removed spark plug wire and checking for a strong, blue spark. 
Symptoms of a Bad Ignition Coil

• Check Engine Light: The vehicle’s computer may detect an issue with the ignition system, triggering the Check Engine Light. 
• Engine Misfires: A misfiring engine often runs rough, coughs, or sputters, which is a common sign of a failing coil. 
• Reduced Fuel Efficiency: A faulty coil may cause the engine to run inefficiently, leading to a decrease in gas mileage. 
• Loss of Power: A failing ignition coil can’t produce a strong enough spark, resulting in a noticeable loss of power and slower acceleration. 
• Difficulty Starting: If the ignition coil isn’t functioning correctly, it can make it harder for the engine to start or even prevent it from starting at all. 
• Engine Backfiring: In some cases, an issue with the ignition coil can cause unburned fuel to ignite in the exhaust system, resulting in backfiring. 

How to Test for a Bad Coil

1. OBD-II Scan: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) from the engine control unit (ECU). 
2. Spark Test:
   • Remove the suspect ignition coil or spark plug wire.
   • Attach a spark tester to the spark plug wire. 
   • Hold the spark tester against a metal part of the engine. 
   • Crank the engine and look for a strong, blue spark across the tester’s gap. No spark, or a weak, orange spark, indicates a faulty coil. 
3. Multimeter Test:
   • Set your multimeter to measure resistance (ohms). 
   • Disconnect the ignition coil and find its terminals. 
   • Consult your vehicle’s repair manual for the manufacturer’s specifications for the coil’s primary and secondary winding resistance. 
   • Test the resistance between the coil’s terminals. If the reading falls outside the specified range, the coil is faulty. 

Important Safety Note: When performing tests that involve the engine’s electrical system, always disconnect the negative battery terminal first to prevent electrical shorts and ensure safety.

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 do I know if the coil on my cylinder head is bad?

5 Signs of a Faulty Ignition Coil

1. It’s Hard to Start Your Car. When there’s a problem under the hood, you’ll typically find it harder to start your car.
2. It’s Impossible to Start Your Car.
3. It’s Taking More Gas to Keep the Car Powered.
4. It’s Harder to Accelerate.
5. It’s Difficult to Drive Without Misfiring.