This O-ring seals the interface between the injector body and the cylinder head in the fuel system. It prevents high-pressure fuel leakage and the ingress of air or contaminants into the injection circuit. A degraded or hardened O-ring causes fuel seepage, which can lead to a loss of injection pressure and rough engine operation. The original manufacturer's material formulation matches the thermal and chemical exposure at the injector location. On replacement, clean the sealing groove and lubricate the O-ring before seating.