These types of tester will only show you that the plug can arc. Since the cylinder pressure in an engine is always changing, you can't be sure how a plug will act under driving conditions. I pulled six plugs out of a Cummin's 8.9L natural gas engine that misfiring on all six cylinders, it was bad enough the engine wouldn't hardly idle. I put all six on my distributor machine and hooked up the wires coming off a Delco points distributor. I could run the rpm all the way up to 6300 and all six plugs fired perfect. Due to the hybrid engines 14:1 compression, the plugs would not fire correctly in the cylinders, but would work in the open atmosphere. Same things happen in less compression, some plugs just won't fire. It's a neat tool, and the guys that used them years ago had plenty of experience with them, but probably could tell you it was bad just looking at the plug without using anything but their eyes and ears.
