What is Corona Stain?

What is Corona Stain, & how does it affect spark plugs performance?

The brown color on white insulation just above metal sheet of spark plugs is called corona stain.

There are famous rumors amongst the circle of untrained auto technicians that this corona stains is the sign of faulty spark plug or a spark plug that has short circuit. This is totally an absurd claim. It has no scientific grounds. If you ask such an auto technician that why this color appears on a spark plug. He will simply say a short circuit. When you ask him again, how? He will be unable to logically answer it. This is because he doesn’t know the science behind this corona stain.

This corona stain is well explained by one of the leading manufacturer of spark plugs; Denso. According to Denso, the suspended particles of oil stick to spark plugs that form brownish color on spark plugs. In more detail, sometimes high voltage in spark plugs produces corona discharge that ionizes gas around spark plugs just above metal sheet. To this are attracted suspended oil particles, and stick firmly, and thus causes brownish color called corona stain.

This stain has no effect on spark plugs performance.