Process of operating the air conditioning system (5) of a cab (1) of a vehicle, said air-conditioning system comprising: a circuit (9) for supplying said cab with air coming at least partially from the exterior of said cab; a blowing device (10) for blowing an air flow inside said circuit; filtration means (11a, 11b) for removing pollutants from the air blown into the cab (1) through said circuit; said process comprising a regular procedure wherein the air-conditioning system (5) is operated to blow the air supplied by said circuit, said process comprising a test of the efficiency of the filtration means (11a, 11b), wherein said test provides for: spraying an identified fluid upstream of the filtration means (11a, 11b) whilst the blowing device (10) is operated to blow an air flow inside the circuit (9); monitoring the air flow supplied by the circuit (9) at a location (22) downstream of the filtration means (11a, 11b) for detection of the possible presence of the sprayed fluid by means of at least one appropriate sensor provided at said downstream location; and providing by means of said sensor a signal representative of the efficiency of said filtration means upon the detection of such a presence.