OTC Tools OTC 3673 Diesel Glow Plug Tester. OTC Tools. Glow Plug Tester provides fault detection through LED's for a quick analysis; Short Circuit, Over Current, Normal Operation, Open Circuit/Under Current. Benefits: Glow Plug Tester allows for testing without removing glow plug from engine. Powers the glow plug to test the plug in it's operating state. Test much more accurate than a cold resistance test, which can miss short circuits. Tester uses a current measurement to accurately determine proper operation and check for short circuits. Email for product availability.