This tool is a "must have" for anyone who works on the 6.7L common rail fuel systems. When a 6.7L engine will not start, one of the first things to check is fuel rail pressure. If the pressure is less than 5,000 PSI, one of the most likely causes is a defective injector that is returning excess fuel back to the tank. A return flow volume test will verify if excessive return flow is the problem. This cap is then used to identify the individual cylinder that is at fault. The injection line is removed for one cylinder at a time and the cap installed on the fuel rail in place of the injection line. When the cylinder with the offending injector is capped, the rail pressure will rise and the truck will start. In some cases it can be highly advantageous to have multiple caps, because several injectors that return slightly too much fuel can have the same effect as a single injector with extremely high return flow. A block off tool can also be used temporarily in the case of a broken injection line or failed injector and is a good thing to have in your emergency kit. When an injector malfunctions on a common rail engine damage to a piston or cylinder can occur very rapidly. When a common rail engine begins to misfire and blow excessive white smoke, the owner is highly advised to turn the engine off to prevent major engine damage from occurring. An alternative would be to temporarily cap the injector for that cylinder for a short period. When working on high pressure fuel systems, always educate yourself first by reading appropriate factory service information, use common sense, and, above all, WEAR SAFETY GOGGLES!