I hope you just aren't understanding my description, because I guarantee you it is correct. A glow plug burning out is just like a light bulb or a fuse, 0 resistance if they are good, infinity resistance if they are bad. If you put one end of a continuity tester (registers Ohlms of resistance, not power) on the threaded (the threaded part I am referring to is where the power connects to the glow plug, not where the glow plug threads into the head) part of the glow plug (the power strip HAS to be disconnected from the glow plug) and the other tester wire to a ground..............if you have 0 resistance, the glow plug is good..............if you have infinity resistance........that glow plug is burned out. You can test a glow plug out of the machine the same way, just touch one probe to the threaded part as described above and the other probe to the body of the glow plug.