The 5-pc Injector Rail Fuel Line Cap Set, No. 10810, from Lisle Corporationhelps in identification of faulty diesel fuel injectors and can be used to diagnose engine knock sounds. Simply install the cap on the fuel rail in the place of the injection line. When the cylinder with the failed injector is capped, the rail pressure will rise and the vehicle will start. The caps are designed for 2003-2022 Cummins, 2001-2022 Duramax, and 2008-2022 Ford engines