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  • Some symptoms of a bad spark plug include an illuminated check engine light, engine misfires, rough idling, and a hard starting condition.
  • You can diagnose the issue is by performing a test or visually inspecting the spark plugs.
  • It’s recommended to change spark plugs between the 20,000 and 30,000 miles range.

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.

5 Symptoms of Bad Spark Plugs – Misfires

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. But on just about every modern vehicle, a steady misfire will cause the check engine light to flash as a warning that catalytic converter damage may be imminent.

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 a fuel injector or wiring issue to an engine mechanical problem can cause misfires.

While almost every vehicle these days will disable the fuel injector on a misfiring cylinder, if the injector is leaking badly enough to cause the misfire, the catalytic converter can be seriously damaged.

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Typically, a misfiring spark plug will cause a “puffing” at the exhaust pipe you can feel by holding a rag where the exhaust comes out. A spark plug that is misfiring sporadically will “puff” sporadically, which typically means the plug is partially fouled or perhaps gapped to close, which causes it to misfire only sometimes.

Misfires that happen only under load may be felt as an irregular “bite” or a flat-out misfire that feels sort of like you’re firing a machine gun if the engine is a four or six cylinder. On a V8 or a V10, a steady misfire under load that doesn’t happen at idle may not be quite so noticeable as on engines with fewer cylinders, and may be due to a spark plug with crack in the ceramic shell that may or may not be visible. It can also be due to a breached or open spark plug wire or even a faulty coil on engines with multiple coils.

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. Once they progress to that point, they’ll typically foul before the vehicle starts in most cases.

To avoid problems such as these, it’s important to replace the plugs at the manufacturer’s recommended service intervals as may be listed in your owner’s manual.

Diagnosing Bad Spark Plugs

replacing car spark plug
It’s important to replace the plugs at the manufacturer’s recommended service intervals as may be listed in your owner’s manual.

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

  1. Check what you can see first without touching anything. Look for spark plug wires resting against the exhaust or running across sharp bracket edges.
  2. With the engine running, spray a mix of mild soapy water on the wires right at the spark plugs and see if the engine begins to misfire. Watch for leaking spark on the cylinder that misfires when you wet the wire. A cracked plug may misfire without a visible spark leak.
  3. Snap the throttle to the floor and very quickly release it. Any sputtering or misfiring is a possible indication of worn spark plugs.
See also  How to Clean a Spark Plug

Note: Do not perform this test unless you have sufficient DIY automotive troubleshooting knowledge and experience.

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.

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.

  • Look for damage, such as cracks or a broken ground electrode.
  • Inspect all spark plugs for possible wear. Usually, if a spark plug is worn out, the central electrode will become rounded. A rounded central electrode will require a higher voltage to fire the spark plug.
  • Generally, all your spark plugs must be in the same condition. The color of the insulator should be light tan or gray. If all your spark plugs are black or dark, this may be due to your engine creating a very rich air-fuel mixture or possible oil burning.
  • If only one or a few of your vehicle’s spark plugs are black, it is recommended to check those cylinders for proper firing. Chances are something may be affecting only those cylinders.
  • A lean running condition or an over advanced ignition timing may cause all your spark plugs to look white or be covered in white. If only a few of your spark plugs are white, you have to check for a vacuum leak that is only affecting those cylinders.
spark plug wet with fuel or engine oil
A spark plug that is wet with fuel or engine oil won’t fire at all. This wetness may be due to the plug itself being faulty, but it can be due to fuel delivery or oil fouling due to deeper engine concerns. Replace the spark plug first to see if the wetness returns. | Image Source: Richard McCuistian
worn out car spark plug
This spark plug came from a 2008 Ford Explorer with 238,000 miles. While this Explorer still ran perfectly, allowing spark plugs to reach this level of wear always strains the other parts of the ignition system. | Image Source: Richard McCuistian
spark plug with hard puffy deposits
A spark plug with hard, puffy deposits means the valve stem seals are leaking. Deposits of this sort may cause surging or random misfiring, but sometimes there may be no symptoms at all, and these deposits take quite a few miles to form. | Image Source: Richard McCuistian

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.

spark plugs in need of replacement
These came out of a 2007 5.4 liter 3 valve Ford Expedition. Do not attempt to remove the spark plugs from one of these engines at home because the shell on the bottom tends to come off and special tools will be required to remove the shell from the head if it happens. Have a shop replace these. | Image Source: Richard McCuistian

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.

See also  Iridium vs. Platinum Spark Plugs

Check out this video on how to replace your spark plugs:

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.

Replacing Worn-Out Spark Plugs

Try not to drive too much if your vehicle has bad spark plugs. At least, not until you’ve made the necessary replacements. Thankfully, you can get a replacement that fits your vehicle with the help of

The best part? You don’t even have to take one step outside your door to get your hands on brand-new spark plugs. All you need to do is visit our website and use our vehicle selector. Just be sure to input the necessary details like the year, make, model, and engine of your vehicle to check out compatible parts.

We make it a point to source our spark plugs from some of the most reliable names in the industry. We also ship out orders from strategically located distribution centers in the US to make sure your orders arrive in a matter of days.

Restore your ignition system’s performance in no time by ordering new spark plugs online. Shop at today!

About The Authors
Written By Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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Patrick kagai kiarie

I live in Kenya,what could be the price of iradium plugs in Nairobi and who are the stockists?

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