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Bad Spark Plug Symptoms

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Spark plugs are vital components of internal combustion engines. They are responsible for producing the “spark” necessary to ignite the engine’s fuel. Although the spark plug is considered vital to engine performance, this humble car part is often ignored until it starts to fail or go bad.

Spark plugs are considered a routine maintenance item because they eventually wear out. Do you remember the last time you had your spark plugs checked or replaced? If you’re starting to notice changes in your vehicle’s engine performance and you think it is related to a bad spark plug, you’re reading the right article.

spark plugs of a car
Spark plugs produce the “spark” necessary to ignite the engine’s fuel.

4 Symptoms of Bad Spark Plugs

Spark plugs are part of your vehicle’s ignition system, so it may be difficult to differentiate one bad ignition part symptom from the other. The best course of action is to let your mechanic diagnose the issue for you.

If you’re the type who knows their way around a car, and you’d prefer to diagnose the issue yourself, here’s how to tell if you’ve got a bad spark plug:

Check Engine Light

Worn-out spark plugs may trigger your check engine light to illuminate. However, because a countless number of other problems can also turn the light on, it may be difficult to say whether one or more of your spark plugs are to blame.

You (or your mechanic) will need to do some troubleshooting to know for sure.

It is recommended to have your vehicle checked by your mechanic regardless of what triggered the check engine light.

Engine Misfire

A worn spark plug may cause engine misfire. However, a misfire may also be a symptom of other faulty components—anything from a bad spark plug to an engine mechanical problem can cause your vehicle to experience misfires.

Keep in mind that a misfire can damage the catalytic converter, so you should address the issue immediately.

starting car engine
One common symptom of bad spark plugs is difficulty starting the car.

Rough Idling

You may experience this symptom as a result of an engine misfire. Spark plugs that are worn-out can significantly reduce the amount of “spark” that is produced to ignite the fuel.

However, take note that this symptom may also be caused by issues in other components. A thorough diagnosis must be performed to know for sure.

Hard Starting Condition

Spark plugs are important to the combustion process. As such, although rare, worn-out plugs can cause you to have difficulty starting your vehicle.

To avoid problems such as these, it’s important to replace the plugs in accordance with the vehicle manufacturer’s service schedule. You’ll find it listed in your owner’s manual.

Diagnosing Bad Spark Plugs

Once you start observing any of the signs listed above, the next step is to perform a diagnosis to help you make sure that your spark plug(s) have gone bad.

Perform a Test

Use a secondary ignition test to detect cracked spark plugs or faulty wirings. To do this, you have to be accompanied by a second person or an assistant.

Note: This test does not work on spark plugs that are recessed in the engine and difficult to see.

If your spark plug is cracked or if the wirings are faulty, a spark should be visible at night.

  1. Your assistant must accelerate the engine with the gear selector set in drive. For manual transmissions, it must be placed in second gear.
  2. If you see any spark visible or if you hear a snapping sound, this may mean something is wrong with one of your ignition components (possibly a spark plug). The location should be inspected and the faulty ignition parts must be replaced right away.

Aside from the steps above, there are also other ways to test your spark plugs and other engine components for faults. However, it is recommended to seek the assistance of a professional when doing any tests, as doing them incorrectly may harm your engine.

old and new spark plugs
Don’t attempt to remove spark plugs if you don’t have enough DIY car repair experience.

Visual Inspection

Visual inspection is an easy way to diagnose whether you have bad spark plugs. Since it’s part of the ignition system, doing this can also give you an idea of what is wrong with the rest of your engine components.

However, you should not attempt to remove your spark plugs if you are not familiar with the process. You can always contact a professional to do the inspection for you.

replacing car spark plug
Spark plugs should last around 20,000 to 30,000 miles, while noble metal plug variants may last up to 120,000 miles.

When to Change Your Spark Plugs

There are different types of spark plugs—copper core, platinum, and iridium. Usually, the type of spark plug installed in your vehicle will depend on your vehicle’s year, make, and model.

The lifespan of a spark plug would depend on the material it’s made of, but generally, it’s supposed to last around 20,000 to 30,000 miles but noble metal plug variants may last until 120,000 miles. Consult the service schedule (listed in your owner’s manual) to determine the recommended replacement interval for your application.

The lifespan of your spark plug will greatly depend on the type of spark plug installed. It is important to watch for symptoms indicating that your spark plug is failing, so you can have a faulty one replaced right away.

Can I Drive With a Bad Spark Plug?

If one or two of your spark plugs is just weak or failing, you may technically be able to drive your vehicle. However, you should address the issue as soon as possible to prevent inadvertently causing damage to other parts of your vehicle.

To avoid going through all these problems, it is best to service your car’s spark plugs at the interval recommended by the vehicle manufacturer.

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In the Garage with CarParts.com is an online blog dedicated to bringing DIYers and devoted car enthusiasts up to date with topical automotive news and lifestyle content. Our writers live and breathe automotive, taking the guess work out of car repairs with how-to content that helps owners get back on the road and keep driving.

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