This disclosure includes systems and methods for testing a vehicle's fuel system. Some systems have a gauge configured to be mounted to a vehicle having a compressed gas fuel system including a compressed gas fuel tank, the gauge having a user interface configured to display fuel pressure within the fuel tank and receive user input to initiate diagnostic testing of the fuel system, and one or more processors configured to, in response to the user input, actuate one or more components of the fuel system, including a solenoid and/or a relay, and display, on the user interface, diagnostic information associated with the actuated one or more components.