This chapter explains how to perform fluid power system malfunction detection and diagnosis. The main function of a hydraulic pump is to move fluids against resistance. The movement of fluid in a hydraulic circuit is incidental to the basic function of power transmission. The function of the pump is to accept liquid at the intake and move it through the pumping mechanism. As the fluid passes through the pumping mechanism, it can be pressurized at the output chamber to a value appropriate to the resistance to flow at the terminus. When testing the hydraulic circuit with the hydraulic tester, first determine how much fluid is flowing in the circuit. Then, by controlling pressure with the load valve, determine the amount of fluid actually flowing through each component. The tester can be installed in the circuit in series, in parallel, or in a bypass arrangement with the valves.