This O-ring seals the injector body to the fuel gallery in the cylinder head. It prevents high-pressure fuel leakage past the injector bore, which would cause a loss of rail pressure and uneven cylinder firing. A degraded or hardened O-ring allows fuel to bypass, leading to hard starting, misfire, or visible external seepage around the injector. The manufacturer machines this seal to match the original groove depth and cross-section, so the injector seats at the correct height without shimming. On reinstall, lubricate the O-ring with clean engine oil and seat the injector squarely to avoid pinching the seal.