Injection assembly of a die-casting machine, comprising: - an injection piston controllable in translation to operate on the cast molten metal in a mold of the machine, and a main pressure chamber for containing and pressurizing the fluid for the translation of the injection piston; - a multiplier piston controlled hydraulically to increase the fluid pressure in the main pressure chamber; - valve means for managing the advancement and the return of the injection piston and of the multiplier piston, wherein said valve means comprise at least one of a proportional feed valve, electronically feedback controlled by a position transducer; an injection return drain valve; an injection return valve; a proportional injection drain valve, electronically feedback controlled by a position transducer; a multiplier release valve; a multiplier return drain valve; a proportional main multiplier valve, electronically feedback controlled by a position transducer; - electronic control means operatively connected to at least one of said valves and to said transducers to control the opening and closing of said valves, configured and programmed according to a predetermined control program comprising a test management program for the performance of a plurality of diagnostic tests on said valves, wherein said test management program provides for the performance of a test for each valve, in sequence one at a time, and the comparison of the parameters detected during said test with a predefined range or with a threshold value; - display means, operatively connected to said management means to display the result of said diagnostic tests. - an injection piston controllable in translation to operate on the cast molten metal in a mold of the machine, and a main pressure chamber for containing and pressurizing the fluid for the translation of the injection piston; - a multiplier piston controlled hydraulically to increase the fluid pressure in the main pressure chamber; - valve means for managing the advancement and the return of the injection piston and of the multiplier piston, wherein said valve means comprise at least one of a proportional feed valve, electronically feedback controlled by a position transducer; an injection return drain valve; an injection return valve; a proportional injection drain valve, electronically feedback controlled by a position transducer; a multiplier release valve; a multiplier return drain valve; a proportional main multiplier valve, electronically feedback controlled by a position transducer; - electronic control means operatively connected to at least one of said valves and to said transducers to control the opening and closing of said valves, configured and programmed according to a predetermined control program comprising a test management program for the performance of a plurality of diagnostic tests on said valves, wherein said test management program provides for the performance of a test for each valve, in sequence one at a time, and the comparison of the parameters detected during said test with a predefined range or with a threshold value; - display means, operatively connected to said management means to display the result of said diagnostic tests.