How To Diagnose A Failing Spark Plug

When one of your spark plugs goes bad, it's time to order replacement spark plugs. And, you can find genuine OEM replacement spark plugs on our website at wholesale pricing! For example, a Subaru Outback spark plug costs $15-26 at a dealership. But you can get these same spark plugs from us for only $10-$20.

How do you know if your car has bad spark plugs, though?

You don't have to bring your car to a shop to diagnose the issue. You can do it right at home! We'll show you how.

Signs Of A Bad Spark Plug

Old new spark plug

If one of your spark plugs goes bad, you'll notice some signs, including:

  • Misfiring
  • Trouble starting your engine
  • Rough idle
  • Inconsistent engine performance (hesitation and surging, for example)
  • Poor fuel economy
  • Sluggish acceleration

These signs are good indicators that one or more of your spark plugs are bad. Yet, you can't confirm it only by noting the signs. It's possible that your spark plugs are fine, but something else within the engine is bad. The only way to confirm that you have at least one bad spark plug is to visually inspect your spark plugs.

The Tools And Equipment To Get First

Replacement spark plug

Diving into your engine to check your spark plugs may sound intimidating. We're happy to say that it's actually a lot easier than it sounds. That, and you don't need any fancy tools. You only need the following tools:

  • Spark plug socket
  • Ratchet

Depending on your car model, you may need a few more basic tools to access your spark plugs. That's pretty rare, though. As long as you have the two tools listed above on hand, you should be fine.

Diagnosing A Bad Spark Plug In 8 Steps

To inspect the spark plugs, you need to access them first. To do this:

  1. Make sure that the engine is off and cool.
  2. Pop the hood.
  3. Find the negative battery terminal and then disconnect it.
  4. Remove the engine cover, if there's one.
  5. Find an ignition coil above one of the spark plugs. Unplug the electrical connector from the ignition coil. Next, remove it from the spark plug.
  6. Now you can see the spark plug. With the spark plug socket and the ratchet, remove the spark plug.
  7. Thoroughly inspect the spark plug. Refer to the next section for a list of things to look for.
  8. Repeat with the rest of the spark plugs in your engine.

What To Look For

First, you want to know what a good spark plug looks like. Obviously, it will have some wear and tear. A spark plug that's in good condition:

  • Has a light tannish-grayish deposit
  • Doesn't have any visible damage such as cracks or erosion

So what does a bad spark plug look like? If you see a spark plug that has visible damage and/or deposits that aren't tannish-grayish, chances are it's bad. To be more specific, look for the following signs of damage:

  • Excessive erosion on the electrode
  • Damage on the porcelain housing (such as cracks)
  • Dry black sooty deposits
  • Wet (oily) black deposits
  • Greenish deposits
  • Whitish deposits

Do you have any questions about diagnosing a bad spark plug? We'll be happy to answer them for you! Just give us a call or fill out this contact form.