13 Symptoms of a Coil Going Bad

Published: 23 Sep 2025

The ignition coil is a crucial component in your car’s engine, responsible for converting the low voltage from the battery into a high-voltage spark to ignite the fuel-air mixture in the cylinders. When an ignition coil starts to malfunction, it can lead to multiple engine problems, affect fuel efficiency, and compromise drivability. Identifying the symptoms early can save you from expensive repairs and ensure your car runs smoothly.

Symptoms of a Bad Ignition Coil

Engine Misfires

A failing ignition coil may not supply enough voltage to the spark plugs, causing one or more cylinders to misfire. Misfires can make your engine stutter, cough, or jerk, particularly during acceleration or when driving under load, creating a noticeable drop in performance.

2. Difficulty Starting the Engine

Faulty ignition coils can make your engine reluctant to start. You may notice extended cranking times, repeated attempts to start, or complete no-start conditions, especially during cold weather, because the spark plugs are not receiving adequate energy.

3. Rough Idling

A bad coil can cause inconsistent spark delivery, leading to rough or uneven idling. You may feel vibrations or shaking when the vehicle is stopped at traffic lights or even notice your engine hesitating during warm-up periods.

4. Loss of Power and Sluggish Acceleration

When the ignition coil fails to produce strong sparks, the engine cannot operate at full efficiency. This can result in reduced acceleration, sluggish response, and a general feeling of underpowered performance, especially when overtaking or climbing hills.

5. Poor Fuel Economy

Incomplete combustion due to weak or intermittent sparks forces the engine to burn more fuel to maintain performance. Over time, this leads to a noticeable drop in miles per gallon and more frequent trips to the gas station, even under normal driving conditions.

6. Check Engine Light

Modern vehicles are equipped with onboard diagnostics that detect ignition misfires or irregular spark patterns. A failing coil can trigger the check engine light, often accompanied by diagnostic trouble codes (DTCs) that indicate specific cylinder misfires or coil issues.

7. Engine Stalling

A damaged coil may cause your engine to stall unexpectedly. This is due to interrupted spark delivery, which can make the engine shut off at idle or while driving at low speeds, creating unsafe driving conditions.

8. Backfiring or Exhaust Popping

Unburned fuel caused by a weak spark can ignite in the exhaust system, creating loud popping noises or backfires. Backfiring may also produce black smoke or fuel odors and can damage the catalytic converter over time.

9. Black Smoke or Strong Fuel Smell

When the spark is weak or inconsistent, fuel may leave the combustion chamber unburned. This often results in black smoke from the exhaust and a noticeable gasoline smell, indicating incomplete combustion caused by a faulty coil.

10. Jerking, Sputtering, or Coughing While Driving

A failing ignition coil can create irregular firing in the engine cylinders, causing your car to jerk, sputter, or cough during acceleration. This not only affects drivability but also signals inefficient combustion that can harm other engine components.

11. Overheating Ignition Coil

Faulty coils can overheat due to electrical stress or prolonged misfiring. Overheating may cause the coil to degrade further, emit a burning smell, or even produce intermittent sparks, which leads to inconsistent engine performance.

12. Engine Vibrations

Multiple weak or failing coils can cause your engine to shake or vibrate excessively. You may notice this particularly when idling, accelerating, or driving at higher speeds, as uneven spark distribution disrupts engine smoothness.

13. Warning Lights Related to Ignition

Besides the check engine light, some cars may illuminate other warning lights, such as misfire indicators or traction control warnings. These alerts often signal ignition system irregularities linked to a faulty coil.

Conclusion

A failing ignition coil can cause engine misfires, rough idling, difficulty starting, loss of power, poor fuel efficiency, stalling, black smoke, backfiring, vibrations, and various warning lights. Identifying these symptoms early and replacing faulty coils ensures smooth engine operation, prevents further damage, and keeps your vehicle safe and reliable. Regular maintenance and high-quality coil replacements can significantly extend the lifespan of your ignition system.

FAQs

How do you know if you have a bad coil?

Symptoms include misfires, rough idling, engine stalling, reduced acceleration, poor fuel economy, check engine light illumination, backfiring, and black smoke from the exhaust.

How do I know when my coils need replacing?

If your car shows multiple ignition-related symptoms, such as repeated misfires, engine hesitation, or diagnostic trouble codes (P0300-P0362), it’s time to replace your coils.

What does a bad coil feel like?

A faulty coil can make your car feel jerky, sluggish, or difficult to start, with vibrations at idle and occasional sputtering or backfiring during driving.

How many years can a coil last?

Ignition coils typically last between 80,000 to 100,000 miles, depending on driving conditions, maintenance habits, and coil quality.