Abstract: Method for error detection in a fuel-supply system includes determining (s410) a prevailing operating status in the engine (280), determining (s415) whether the determined operating status corresponds to a predetermined operating status, which operating status is regarded as stationary; continuously determining (s420) a progression in the fuel-supply pressure (P_rail) during the prevailing operating status; determining (s430) the number of occasions (N) within a specified period (DT) that the fuel-supply pressure (P_rail) alternately reaches a maximum or minimum value that deviates from a predetermined reference value (P_rail_ref) by more than a predetermined deviation value; and if the number of occasions (N) exceeds a specified value (TH), this indicates that the valve device (250) is not functioning as intended. Also a computer program product comprising program code (P) for a computer (200; 210) for implementing the method. Also a device and a motor vehicle equipped with the device.