Normal appearance

White-grey discoloration of insulator and some deposits.

Deposits

With mechanically worn engines deposits are forming on the spark plug and allow glow ignition.

Insulator breakage

Ceramic breakage from mechanical cause (installation, transport, impact) or thermal shock.

Melting

Caused by detonation / pre-ignition.

Carbon deposit

Caused by detrimental operating conditions or too cold heat range.

Spark-over

Voltage flash-over due to moisture or dirt.

Corona spot

Normal brown discoloration due to deposits of electrically charged oil particles. No indication of malfunction or leakage.