A diagnostic method for an oil piston cooling jet valve of an oil pressure system of an internal combustion engine of a motor vehicle, including: Driving the OPCJ valve for detecting oil pressure diagnostic data if multiple enabling conditions have been met, the multiple enabling conditions including: Presence of a stationary operation of the internal combustion engine; presence of a predetermined operating range; presence of an oil temperature within a predetermined oil temperature range; prevention of a scheduled driving of the OPCJ valve for regular piston cooling; and prevention of an error of the oil pressure system. The diagnostic method further including: Determining whether an oil pressure measuring point is within a predefined oil pressure measuring point range. The invention further relates to a diagnostic device, a control unit and a motor vehicle, which are each suitable or configured for carrying out the method.