A used spark plug is a window into your engine. The color and condition of the electrode can reveal fuel mixture problems, oil consumption, coolant leaks, and more — all without expensive diagnostic equipment. Here's how to read what you're looking at.
Normal (Good)
Appearance: Light tan or grayish-brown electrode, minimal deposits, slight electrode wear.
Meaning: Engine is running at the correct air-fuel ratio and operating temperature. This is what you want to see.
Carbon Fouling (Black and Sooty)
Appearance: Dry, fluffy black carbon deposits on the electrode and insulator.
Meaning: Rich fuel mixture, clogged air filter, or lots of short-trip driving. The engine isn't burning fuel completely.
Action: Check air filter, inspect fuel injectors, evaluate driving habits.
Oil Fouling (Black and Wet)
Appearance: Wet, oily deposits on the plug, often dark brown or black.
Meaning: Oil is getting into the combustion chamber — worn piston rings, valve seals, or turbo seals.
Action: This is a sign of a more serious mechanical issue. Have the engine inspected.
Overheating (White or Blistered)
Appearance: White, chalky deposits, melted or eroded electrode, blistered insulator.
Meaning: Plug is running too hot — wrong heat range, lean fuel mixture, or engine overheating.
Action: Check coolant level, thermostat, and fuel mixture. Verify you have the correct plug heat range.
Normal Wear (Eroded Electrode)
Appearance: Rounded, eroded center electrode with no deposits.
Meaning: Plug has reached end of service life through normal wear. Replace it.
Find replacement plugs at Texan Supply — free shipping.