The invention relates to a method for diagnosing a sensor (16, 17) of a motor vehicle (10) designed to detect road infrastructures (101, 102, 103, 104), the said motor vehicle comprising a means of communication (18) designed to communicate with a remote server (50) and a computer (14) connected to the sensor and to the means of communication. According to the invention, the diagnostic method comprises steps during which: a) the computer identifies an infrastructure and assigns it an effective score, which relates to the visibility of this infrastructure, b) the remote server acquires a reference score which is assigned to the said infrastructure and which relates to the visibility of this infrastructure, and c) the effective score and the reference score are compared so as to deduce from this a state of operation of the sensor.