What Are the Symptoms of Bad Spark Plugs?

Your Car Might Be Telling You Something—Don’t Ignore These Signs

Spark plugs might be small, but they play a big role in keeping your engine running smoothly. Without healthy spark plugs, your vehicle can suffer from poor performance, lower fuel economy, and even long-term engine damage. If you’ve noticed your car acting differently lately, worn or faulty spark plugs could be to blame.

In this post, we’ll break down the most common symptoms of bad spark plugs, why they matter, and what to do if your vehicle shows signs of trouble.

1. Rough Idling or Engine Misfires

One of the earliest and most noticeable signs of bad spark plugs is a rough idle. If your car feels shaky when stopped or the engine stumbles while running, your spark plugs may not be delivering the proper spark to ignite the fuel-air mixture.

Common symptoms include:

• Shaky or vibrating engine at idle

• Jerky movements or stuttering when accelerating

• A popping or knocking sound from the engine

If left unchecked, misfires can lead to more serious engine damage over time.

2. Trouble Starting the Car

Your spark plugs are responsible for getting the engine fired up. Worn or fouled plugs may struggle to produce a strong enough spark to start the engine efficiently. If your car is slow to start, especially on cold mornings, don’t just blame the weather—it could be your spark plugs.

3. Reduced Fuel Efficiency

Bad spark plugs can reduce your miles per gallon because the engine works harder to produce the same amount of power. According to the U.S. Department of Energy, replacing worn spark plugs can improve fuel economy by up to 30% in some cases.

Look out for:

• Declining MPG over time

• Filling up more frequently with no changes to your driving habits

4. Lack of Acceleration or Power

Does your vehicle hesitate when you step on the gas? Faulty spark plugs may fail to ignite fuel properly, resulting in poor engine performance and sluggish acceleration. If your engine feels like it’s not responding the way it should, spark plugs are a common culprit.

5. Check Engine Light Is On

Modern vehicles use onboard diagnostics to monitor engine performance. A misfire or poor combustion caused by bad spark plugs can trigger your check engine light. While a code reader can confirm the cause, this is a clear sign not to ignore.

6. Engine Surging or Hesitation

If the engine feels like it's pulling or surging unexpectedly while driving, it could be due to inconsistent firing from damaged spark plugs. Surging usually feels like a sudden increase in speed or power when you're not pressing harder on the gas pedal.

How Often Should Spark Plugs Be Replaced?

Most vehicles need new spark plugs every 30,000 to 100,000 miles, depending on the type (copper, platinum, or iridium) and your driving conditions. Always check your owner's manual for manufacturer recommendations.

Don’t Ignore the Warning Signs

While spark plugs are a relatively low-cost fix, ignoring the symptoms can lead to more expensive repairs like damaged catalytic converters or ignition coils. If you're experiencing any of the above symptoms, it’s worth having a professional inspect your spark plugs and ignition system.

Final Tip

If you’re noticing these issues and also see more carbon build-up or engine grime, it could be a sign your vehicle needs more than just a tune-up. Pair spark plug maintenance with regular detailing and engine bay cleaning to extend the life of your vehicle.