A monitor for a programmed sequence of operations. A sequence controller responds to successive instructions. Some instructions correspond to elements of governing functions and test the status of independent variables to activate controllable devices, which correspond to dependent variables, when functions are satisfied. The monitor, operating in synchronism with the controller, generates a transition word each time the controller either (1) tests a sensor for an independent variable in the function or (2) prepares to alter a controllable device, if a comparison with the last condition of that sensor or device in a memory unit indicates a change has occurred or will occur. The monitor also updates this memory unit to the new condition. Each transition word identifies the changing element and describes the change. A data processing system stores each transition word in sequence for comparison with a reference list of transition sequences.