Reading a Spark Plug: What the Tip Color Tells You

Pull a plug and look at the firing tip. Its color and condition is one of the oldest and most reliable diagnostic readouts in the shop.

Light tan / grayish-brown — healthy

This is what you want: a light tan or gray insulator and minimal deposits. It means the engine, fuel mixture and heat range are all correct.

Black and dry (sooty) — running rich

Dry black soot means too much fuel or too little air — a rich mixture, a dirty air filter, or a plug that's too cold for the engine. Fouls cause misfires.

Black and oily (wet) — oil in the chamber

A wet, oily deposit points to oil getting past valve seals or piston rings. This is a mechanical issue, not just a plug issue, and worth investigating.

White or blistered — running too hot

A chalky white insulator or a blistered tip signals overheating: too-lean mixture, over-advanced timing, or a plug heat range that's too hot. This one can lead to engine damage if ignored.

Melted or broken electrode — pre-ignition / detonation

A melted center electrode or physical damage is a red flag for pre-ignition or detonation. Stop and diagnose before more harm is done.