A diagnosis method for the diagnosis of the correct function of a heating and/or cooling system with at least one load circuit (6), through which a fluid flows as a heat transfer medium. For the diagnosis an opening degree (Vpos,n) of the load circuit (6) is changed for changing the flow. Subsequently a differential pressure (Hpu) across the load circuit (6) and/or a volume flow of a fluid flowing through the load circuit (6) is detected and the detected values, or at least a value derived from the detected values, are/is compared to at least one predefined system limit value.