How To Diagnose A Fouled Spark Plug In Your Subaru

It’s normal for spark plugs to wear out over time, especially after 40,000 miles. When a spark plug wears out, your engine’s performance suffers and fuel consumption gets worse.

Symptoms Of A Fouled Spark Plug

The first step towards better performance and fuel economy is to recognize the signs of a bad spark plug. Some of the most common symptoms include:

  • Engine misfires
  • Sluggish acceleration
  • Rough idle
  • Trouble starting your car

Checking Your Spark Plugs In 7 Steps

Old and new Subaru spark plugs

Image Credit: CarsNToys

To confirm your suspicions, you need to physically examine your spark plugs. To do this:

  1. Open the hood.
  2. Remove the engine cover (if there’s one).
  3. Remove the air filter housing.
  4. Disconnect the ignition coil from one of the spark plugs.
  5. Unscrew the spark plug and then pull it out of the cylinder.
  6. Inspect the spark plug.*
  7. Repeat with the rest of the spark plugs in the engine.

*While examining the spark plugs, look for any of the following issues:

  • Excessive black oily or sooty deposits
  • A “wet” appearance
  • Heavy green or white deposits
  • Excessive erosion at the J-shaped electrode (meaning the thickness isn’t the same at the base and the tip)

If you notice any of these issues, then you’ve got a bad spark plug and it needs to be replaced as soon as possible.

What To Do If One Of Your Spark Plugs Is Bad

Subaru oem spark plug

You want to replace a fouled spark plug as soon as possible. Even if you only have one bad plug, we recommend replacing all of them at the same time. If one plug has gone bad, the others don't have long to live. You can order an OEM replacement spark plugs online at a discounted price instead of visiting a Subaru dealership and paying a 30% markup. At, we offer wholesale pricing on all genuine OEM Subaru parts, including spark plugs.

For instructions on replacing your spark plugs, check out this tutorial we put together for Subaru vehicles.