Method for detecting a malfunction of a dual-sensing system for sensing a fluid mixture stored in a tank of a vehicle, the dual-sensing system being able to provide values of two physical quantities, i.e. one quantity indicative of a concentration of a constituent of the fluid mixture within the tank and one quantity indicative of a level of the fluid mixture within the tank, the dual-sensing system comprising: - a first ultrasound subsystem for determining the value of the physical quantity indicative of the concentration of the constituent of the fluid mixture; and - a second ultrasound subsystem for determining, based on the value provided by the first ultrasound subsystem, the value of the physical quantity indicative of the level of the fluid mixture within the tank, the method comprises the steps of: - when key is off: memorizing the last known value of a first physical quantity out of the two physical quantities and the last known value of a second physical quantity out of the two physical quantities; - when key is on, sequentially: - having new values of the two physical quantities provided by the dual-sensing system; - if the last known value of the first physical quantity is not the same as the new value of the first physical quantity, stop the carrying out of the method; - If the last known value of the first physical quantity is the same as the new value of the first physical quantity and if the last known value of the second physical quantity is not the same as the new value of the second physical quantity, emitting a signal indicative of malfunctioning. - a first ultrasound subsystem for determining the value of the physical quantity indicative of the concentration of the constituent of the fluid mixture; and - a second ultrasound subsystem for determining, based on the value provided by the first ultrasound subsystem, the value of the physical quantity indicative of the level of the fluid mixture within the tank, - when key is off: memorizing the last known value of a first physical quantity out of the two physical quantities and the last known value of a second physical quantity out of the two physical quantities; - when key is on, sequentially: - having new values of the two physical quantities provided by the dual-sensing system; - if the last known value of the first physical quantity is not the same as the new value of the first physical quantity, stop the carrying out of the method; - If the last known value of the first physical quantity is the same as the new value of the first physical quantity and if the last known value of the second physical quantity is not the same as the new value of the second physical quantity, emitting a signal indicative of malfunctioning. - having new values of the two physical quantities provided by the dual-sensing system; - if the last known value of the first physical quantity is not the same as the new value of the first physical quantity, stop the carrying out of the method; - If the last known value of the first physical quantity is the same as the new value of the first physical quantity and if the last known value of the second physical quantity is not the same as the new value of the second physical quantity, emitting a signal indicative of malfunctioning.