A method and apparatus are disclosed for testing a valve control system in an aircraft fuel supply system having a plurality of control valves, wherein the control system includes relay means for outputting control signals to each of the plurality of control valves. The method includes providing connection means for electrically connecting a test device to the control system relay means, connecting a test device to the relay means such that the test device is adapted to receive control signals output by the control system to any one of the plurality of control valves, detecting an electrical control signal output from the relay means to any one of the plurality of control valves, and identifying the particular one of the plurality of control valves to which a detected control signal is output by the relay means and identifying the nature of the control signal output to that particular control valve.