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How to Diagnose a Bad Spark Plug

Spark plugs are pretty important parts of your engine. They are located in the cylinder head directly above each cylinder. Spark plugs are responsible for providing the ignition needed for the combustion process to start.

Like most car parts, spark plugs eventually wear out. When you suspect that one of your spark plugs isn’t working quite right, you should confirm it before it snowballs into a bigger issue.

Symptoms to Look For

Replace spark plug

Image Credit: Paul Miguels

It’s important to catch a bad spark plug early on. A bad spark plug will drastically reduce your engine’s performance and make it eat up more gas than necessary. If you experience some or all of the following symptoms, there’s a good chance one of your spark plugs has fizzled out and it needs to be replaced.

  • Difficulty starting the engine
  • Sluggish acceleration
  • High fuel consumption
  • Rough idle

The Diagnostic Process

Wondering if one your spark plugs has gone bad? It’s time to confirm your suspicions and pop the hood to take a look at your spark plugs! It’s easy to identify a bad spark plug from a used one in good condition. We’ll show you how, but first here’s a basic overview of how to access your spark plugs.

The first thing you want to do is to gather together all the tools needed to make the job go as smoothly as possible:

  • 10mm socket
  • Spark plug socket
  • Socket extension (7” or 10”)
  • Ratchet wrench

To access your spark plugs, take the following steps:

  1. Open the hood.
  2. With the 10mm socket, remove the engine cover.
  3. Remove the bolt holding the ignition coil down.
  4. Pull up the ignition coil.
  5. Attach the spark plug socket to the socket extension and then remove the spark plug.
  6. Examine the spark plug to see if it’s still good.
  7. Repeat with the rest of the spark plugs.

Identifying a Bad Spark Plug

Compared to a good spark plug, a fouled spark plug is:

  • Covered in oil or soot
  • Melted at the top
  • Excessively eroded at the top
  • Cracked anywhere in the housing

If your spark plug doesn’t have any of the issues listed above, but it has a light grayish-tan deposit, then it’s still good. That kind of wear and tear is completely normal for spark plugs.

Finding a Genuine OEM Replacement Spark Plug

Toyota plugs

If you’ve determined that your Toyota has a bad spark plug (or two), you’ll have to replace it as soon as possible. Luckily, it’s a pretty easy and straightforward process.So there’s really no point in having a repair shop tackle the project for you. All you need is this tutorial and a replacement spark plug for each bad one you have.

When it comes to replacement spark plugs, you have two options: OEM or aftermarket spark plugs. OEM spark plugs are the far superior option because of the reasons listed here.

If you’re wondering where to find spark plugs, we have a catalog full of genuine OEM spark plugs at discounted prices. Feel free to contact us if you need assistance finding the right spark plug for your vehicle!

Written by Jason Lancaster