The cylinder leakage test can show a leaking cylinder head gasket. If coolant is getting into the cylinder, burning it washes the cylinder clean and can lead to the spark plug insulator being a lighter color than normal.
You also have to look at fuel trim numbers on the live data display on a scanner. If they're high positive, it means the computer is seeing the need to add fuel beyond the normal calculated values. A lean condition can cause a cylinder to run hotter than normal and lead to the light-colored spark plug insulator. High negative numbers means the computer is subtracting fuel from the pre-programmed values in an effort to fix the rich condition, but perhaps without success.
Your mechanic will introduce an artificial lean and rich condition to see how the computer responds. There are many things that can cause a rich or lean condition to be detected while the opposite is occurring in the cylinder. That's why diagnostic fault codes are just the starting point. He may also short out each spark plug wire momentarily to see what effect that has on what the oxygen sensors are reporting. The problem is all of these things require a scanner to see what is taking place.
Wednesday, April 22nd, 2015 AT 4:35 PM