Main relay operates when Ignition switch is "ON" and it supplies power to ECM, fuel pump relay, CMP, EGR actuator, throttle flap actuator, PTC heater relay and brake switch, etc. Especially, the power supplied to ECM is used as a main power of injectors, rail pressure control valve and ECM operation.When IG KEY ON signal is shut off during turning engine OFF, ECM stops injector operation then engine turns OFF. Approx. after 5 sec., ECM shuts OFF main relay and system turns OFF.Main relay is very important for engine operation, so careful inspection is strongly required.