testing should be completed with a volt meter checking the resistance. Yes the battery test will tell you if there is still connectivity inside the glow plug but it will not tell you the condition of the glow plug itself. I believe the nominal resistance should be less than one ohm for a good glow plug. If you get more than that the filament that is inside is likely starting to burn out and will fail soon. It WILL still glow if this is the case just not as it should.

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2003 Dodge Ram 2500 Cummins - Georend fully built trans, Banks Big Hoss Bundle, Smarty Tuner, 5" turbo back exhaust. 500 HP and 1100 ft/lbs to the ground.
1979 Mercedes-Benz 300CD - Lost to a four car pile up on 275, Thank You Michigan Drivers.
1986 Mercedes-Benz 300D - Sold, Euro non-turbo car.