Remove the spark plug and note its cylinder location
Check the electrode color
Light tan or gray indicates normal combustion
Black, dry soot indicates a rich fuel mixture or weak spark
Black, oily deposits indicate oil burning
White or blistered deposits indicate overheating or a lean mixture
Check for wet fuel on the plug
Wet gasoline indicates no ignition or flooding
Inspect the center and ground electrodes
Rounded or heavily worn electrodes indicate age and replacement need
Look for cracks in the ceramic insulator
Check for melted, broken, or missing electrode material
Measure the gap if needed
Compare the plug condition across all cylinders
Match the plug type and condition to the engine’s symptoms
Replace the plug if it is fouled, damaged, or worn