What are the symptoms of a bad ignition coil?

Common signs that an ignition coil is failing include engine misfires, a rough idle, trouble starting the car, reduced fuel economy, and a check engine light.

Ignition coils are responsible for delivering the spark that starts each engine cylinder. When a coil deteriorates, the resulting weak or intermittent spark can cause one or more cylinders not to fire properly, leading to drivability problems and higher emissions. This article explains the most frequent symptoms and how they typically present in modern vehicles, including coil-on-plug and distributorless systems.

Most common symptoms

Here are the signs you’re most likely to notice if a coil is failing. They can vary by engine type and may resemble issues caused by spark plugs, wiring, or fuel delivery, so a proper diagnostic is important.

• Engine misfires or hesitates under load


• Rough idle or vibrations when the engine is running


• Difficulty starting the engine or a no-start condition


• Reduced power and sluggish acceleration


• Decreased fuel economy and higher fuel consumption


• Check Engine Light with related diagnostic codes (for example P0300 for random misfires or P0350–P0356 for ignition coil faults)


• Engine stalling or surging, particularly at low speeds


• Backfiring or popping sounds from the exhaust

Because symptoms can overlap with other ignition or fuel system problems, a professional diagnostic using a scan tool, cylinder drop test, or coil resistance test is recommended to confirm a failing coil and identify the exact cylinder affected.

When to seek repair and next steps

If you notice persistent or worsening signs, schedule service promptly. A failing coil not only hurts performance and efficiency but can also stress the catalytic converter and downstream sensors. A mechanic will typically check codes, inspect spark plugs and wires, test coil primary/secondary resistance, and replace the faulty coil(s) as needed.

Summary

Bad ignition coils most often reveal themselves through misfires, rough idling, starting problems, power loss, and a check engine light. Acting quickly to diagnose and replace a faulty coil can restore smooth running, protect emissions systems, and improve overall engine reliability.

How to tell if a coil needs replacing?

You can tell a coil needs replacing by observing symptoms like poor performance, such as a burnt taste from a vape or engine misfires in a car. Other signs include a decrease in vapor or power, an illuminated "check engine" light, leaking, or unusual sounds like gurgling or backfiring. A visual inspection for physical damage like cracks, and using a diagnostic scanner to check for specific codes can also confirm a bad coil. 
For a vehicle ignition coil

• Check Engine Light: An illuminated light on the dashboard is a common indicator of an ignition system issue. 
• Engine Misfires: You may feel the engine shudder, jerk, or run unevenly, especially during acceleration. 
• Rough Idle: The engine may shake or vibrate noticeably when the car is stopped. 
• Difficulty Starting: The engine may take longer to crank before starting or might not start at all. 
• Reduced Power: You might notice a loss of power when accelerating or a general sluggishness. 
• Poor Fuel Economy: The engine may burn fuel inefficiently, leading to worse gas mileage. 
• Backfiring: A sudden backfire (a loud pop) can occur if unburnt fuel ignites in the exhaust system. 
• Visual Damage: Look for physical damage like cracks in the coil's housing or oily residue around it. 
• Diagnostic Codes: An OBD-II scanner can read trouble codes (like P0300) that point to a specific cylinder misfire. 

For a vape coil

• Burnt Taste: This is the most common sign, as the cotton wicks and coil element wear out over time. 
• Diminished Flavor: Your e-liquid may not taste as good or the flavor profile becomes weaker. 
• Reduced Vapor: Your device produces less vapor than it used to. 
• Gurgling Sounds: A hollow bubbling sound can indicate the coil is having trouble and needs replacing. 
• Leaking: A worn-out coil can cause the e-liquid to leak from the device. 
• Darker E-juice: The color of your e-liquid might darken prematurely. 
• Airflow Clogging: You may notice the airflow feels restricted or clogged. 

How can you tell when an ignition coil is bad?

A bad ignition coil can be identified by symptoms like a flashing check engine light, engine misfires (causing jerking, sputtering, or a rough idle), sluggish acceleration, and difficulty starting the engine. You may also notice poor fuel economy or hear a loud backfiring sound from the exhaust, which can happen when unburned fuel ignites.
 
Common symptoms of a bad ignition coil 

• Check engine light: A check engine light on your dashboard is a common sign that the car's computer has detected a problem, often an ignition system issue. 
• Engine misfires: This can feel like a coughing or sputtering sound, a strong vibration, or a jerking sensation, especially when accelerating. 
• Rough or shaky idle: The engine may run unevenly and vibrate when the vehicle is stopped or idling. 
• Sluggish acceleration: The vehicle may hesitate and feel slow to accelerate because the engine isn't getting enough power. 
• Difficulty starting: The car may crank for a long time before starting, or fail to start altogether. 
• Poor fuel economy: The engine may use more fuel than usual because it is not combusting efficiently. 
• Backfiring: A loud bang from the exhaust is caused by unburned fuel entering and igniting in the exhaust system. 

How to confirm (advanced)

• Use an OBD-II scanner: A mechanic can use this tool to read specific misfire codes (e.g., P0300-P0308) that pinpoint which cylinder is misfiring, providing strong evidence of a faulty coil. 
• Test resistance: A mechanic can measure the resistance of the ignition coil with a multimeter to see if it falls within the manufacturer's specified range. A reading of zero ohms often indicates a shorted coil. 

What does a car do when it has a bad coil?

One of the most common symptoms associated with a faulty ignition coil is engine performance issues... Faulty coils may cause the vehicle to experience misfires, a rough idle, a loss in power and acceleration, and a reduction in gas mileage. In some cases the performance issues may even result in the vehicle stalling.

How often should you replace ignition coils?

You should replace ignition coils when they fail, not on a regular maintenance schedule, though they often last between 60,000 and 100,000 miles. The best approach is to replace a coil only after it has failed, which is indicated by signs like engine misfires, rough idling, decreased fuel efficiency, or difficulty starting. 
When to replace

• When a coil fails: Replacement is necessary when a specific coil fails and causes performance issues. 
• Proactive replacement: While not a routine maintenance item, some people replace a full set of coils around 100,000 miles, especially if you have reason to believe other components are also nearing the end of their lifespan. 

Signs of a failing coil

• Engine misfires: The engine stutters or runs roughly. 
• Rough idling: The engine shakes or idles unevenly. 
• Decreased fuel efficiency: You may notice a drop in gas mileage. 
• Difficulty starting: The engine takes longer to crank or struggles to start. 
• Check engine light: A fault code is often stored when a coil is not working properly. 

This video explains the signs of a failing ignition coil: 1mHonest Mechanic ColoradoYouTube · Dec 27, 2023
Important considerations

• Quality: The quality of replacement coils can vary, so choose reputable brands. 
• Maintenance: Regularly replacing spark plugs helps keep the ignition coils healthy. 
• Consult your manual: Always check your car's owner's manual for specific maintenance recommendations from the manufacturer. 

You can watch this video to learn more about ignition coils and whether you should replace them with spark plugs: 47sHonest Mechanic ColoradoYouTube · Mar 2, 2024