A diagnostic method for diagnosing a malfunction of a solenoid valve includes: a) storing reference features of a solenoid current waveform, b) detecting features of the solenoid current waveform during operation of the solenoid valve, c) comparing the reference features with the detected features, d) comparing the pressure value of fluid entering the solenoid valve inlet orifice with a predetermined range of pressure values in the event of deviation between the value of the detected features and the value of the reference features, e) adjusting the pressure value and repeating steps b) and c) in the case of deviation of the pressure value with respect to the pressure value range, and f) generating an alarm signal due to malfunction of the solenoid valve in the event of a deviation between the value of the detected features and the value of the reference and fluid pressure features within the pressure value range.