Method for protecting against frost a system installed in a motor vehicle comprising a liquid agent and, in a first so-called active state, delivering the liquid agent when the motor vehicle is making a journey, the liquid agent being liable to freeze at a freezing temperature (Tfreeze), the system, in a second so-called inactive state, ceasing to deliver the liquid agent after the vehicle is stopped, the method comprising the following steps: - measurement, at a predefined pitch between two consecutive measurement points (P1 to P5), and then storage, of external temperature values (T1 to T5) at the various measurement points (P1 to P5) during the last journey of the vehicle, - at the time of a restarting of the vehicle following the last journey, selection of an external temperature value (Tselec) stored during the last journey or of a value that is a function of at least one external temperature value stored during the last journey and comparison of the selected stored value (Tselec) or of the value that is a function of at least one stored external temperature value with the freezing temperature (Tfreeze), - if the selected stored value (Tselec) or of the value that is a function of at least one stored external temperature value is higher than the freezing temperature (Tfreeze), activation of the system in its first state, - if the selected stored value (Tselec) or of the value that is a function of at least one stored external temperature value is lower than the freezing temperature (Tgel), detection of a risk of freezing of the liquid agent and maintenance of or passage into the second inactive state of the system for a predetermined period, - after elapse of the predetermined period, measurement of the external temperature (Text) then, with, if the external temperature (Text) is lower than the freezing temperature (Tgel), maintenance of the system in the inactive state as long as the external temperature (Text) remains below the freezing temperature (Tgel). - measurement, at a predefined pitch between two consecutive measurement points (P1 to P5), and then storage, of external temperature values (T1 to T5) at the various measurement points (P1 to P5) during the last journey of the vehicle, - at the time of a restarting of the vehicle following the last journey, selection of an external temperature value (Tselec) stored during the last journey or of a value that is a function of at least one external temperature value stored during the last journey and comparison of the selected stored value (Tselec) or of the value that is a function of at least one stored external temperature value with the freezing temperature (Tfreeze), - if the selected stored value (Tselec) or of the value that is a function of at least one stored external temperature value is higher than the freezing temperature (Tfreeze), activation of the system in its first state, - if the selected stored value (Tselec) or of the value that is a function of at least one stored external temperature value is lower than the freezing temperature (Tgel), detection of a risk of freezing of the liquid agent and maintenance of or passage into the second inactive state of the system for a predetermined period, - after elapse of the predetermined period, measurement of the external temperature (Text) then, with, if the external temperature (Text) is lower than the freezing temperature (Tgel), maintenance of the system in the inactive state as long as the external temperature (Text) remains below the freezing temperature (Tgel).