This diesel fuel injector cap block off tool is a "must have" tool for everyone working on common rail fuel systems. When the pressure of the common rail is too low to start the engine, one of the possible reasons is a defective diesel fuel injector returning excess fuel back to the tank. To identify the individual defective cylinder, you can remove the diesel fuel injection line from the cylinder, and use this diesel fuel injector block off tool instead of diesel fuel injection line to seal common rail pipe. If the engine can be started normally, and the common rail pressure return to normal value, it's to say that the injector of this cylinder is malfunction. And more, if your scanner cannot perform certain tests, this diesel injector block off tool can also be used for diagnosing engine knocking sounds that could possibly be caused by a single cylinder issue. Package Quantity: 2