How Spark Plugs Work

Troubleshooting Spark Plugs

Removing old spark plugs can tell you a lot about your car's condition, especially around the electrodes at the tip, where the spark happens. This is what those old mechanics mean when they talk about "reading" the spark plug. Don't worry, though: It's a lot less mystical than gazing into a crystal ball.

Reading a spark plug can turn up a slew of issues, and we'll cover the most common ones here. For instance, if the spark plug only looks a little dirty, it's okay. That spark plug has been doing its job for at least 30,000 miles (48,280 km). If it looks glossy, the spark plug may be overheating. If it's white, the spark plug is too hot for the engine. In other words, it has too much insulation to burn away those deposits. That's easily remedied with a colder, more appropriate spark plug. If the old plug looks oily, you've got problems: Oil is somehow getting into the combustion chamber, where it doesn't belong. Spark plugs can be a good indicator of this, but new plugs aren't the solution.

Stop if you find that the spark plugs are sitting so tight in their holes that you can't wrench them out. Breaking a spark plug off in the cylinder head means you've got to drill it out or make another trip to the store for a special tool to remove the broken part. In the worst case, the engine's cylinder head would have to be pulled and taken to a shop for repair. Sometimes you can avoid these headaches by using a penetrating oil like WD-40. Let it sit for an hour or two and try unscrewing the plugs again. When you replace the plug with a new one, remember to use an anti-seize material like Thread Magic to keep from having the same problem in 30,000 miles (48,280 km).

Sometimes after replacing the spark plugs, you may find that the car backfires or runs really rough or won't start at all. This is usually because you didn't follow the handy tips above and you've put the spark plug wires back on in the wrong order. Check your owner's manual for the solution to the mistake.

For more information about spark plugs and other automotive parts, please see the links below.

Related HowStuffWorks Articles


  • "Spark Plug Basics." (accessed July 6, 2008)
  • "The Fuel and Engine Bible." (accessed June 23, 2008)
  • Car Craft. "Choosing the Right Spark Plug for Your Custom Engine." accessed July 14, 2008
  • "Dear Tom and Ray." Feb. 10, 1996. (accessed June 26, 2008)
  • Diesel Technology Forum. "Glow Plugs." (accessed July 6, 2008)
  • Ganesan, V. "Internal Combustion Engines." McGraw-Hill Inc. 1994.
  • How to Change Spark Plugs. "How to change your spark plugs in 10 easy steps." (accessed June 26, 2008)
  • NGK Spark Plugs. "How Do I Read a Spark Plug?" (accessed July 6, 2008)
  • NGK Spark Plugs. "Tech Info - Spark Plugs Overview." (accessed July 6, 2008)
  • Old Burb Club. "Interpreting Your Spark Plugs." (accessed June 25, 2008)
  • "Spark Plug 411." (accessed July 6, 2008)