Over time, the spark plugs in a healthy engine develop a slight brown or tan tint of color on the center and ground electrodes. The electrode wear is uniform and like the threads, should appear free of oil and unburned carbon. Here's a guide on how to detect issues by the color of deposits found on your spark plug.

Normal

What to Look For

Brown or grayish-tan deposits on the side of the electrode.

What This Means

Everything is fine with the spark plug and it should continue to deliver dependable performance.

Note: It is common if you add "off-the-shelf" fuel additives i.e. fuel injector cleaners into your vehicle, this can leave brightly colored residue on the plug.  

Even though the deposits you are seeing suggest the plugs are operating normally, it is recommended that you verify the plug gap is set correctly prior to reinstalling the spark plugs.

Ash Deposits

What to Look For

Light brown deposits encrusted on the center and/or ground electrode. These deposits are caused by excessive oil, or fuel additives.

What This Means

If the deposits are on one side of the plug, this is an indication of upper engine wear (valves, seals, cylinder head). If the deposits appear oil around the electrode, this usually means lower engine wear (cylinder, pistons). 

Oil/Fuel Fouled

What to Look For

Oil fouling occurs when oil is allowed to enter the combustion chamber.

What This Means

Oil deposits cover the spark plug which can keep the spark from arcing across the gap. Instead it takes the shorter path to ground through the oil. Usually a sign of advanced engine wear. 

Carbon Fouled

What to Look For

Soft, black, sooty dry deposits on plug indicate carbon fouling.

What This Means

Carbon fouling is an indication of a rich air-fuel mixture, weak ignition, or improper heat range (too cold). Carbon deposits are conductive and can create a path for spark plug misfire. 

Too Hot

What to Look For

Chalky white insulator (with no tan coloring) pitted or blistered electrodes indicate the plug is running too hot. In some cases the insulator will begin to turn gray or dark blue.

What This Means

Improper heat range plug, lean air fuel mixture, or ignition timing may cause this condition. Pay attention to exhaust restrictions or engine overheating which may also cause plug overheating.

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