receiving temperature sensor information at predetermined periodic intervals from a temperature sensor transmitting air vent temperature information at an output air vent of an air conditioning system within a vehicle passenger compartment;

actuating refrigerant filling into the vehicle coolant system;

determining from the temperature sensor information an initial negative change in temperature (ΔTinitial) from a first time interval (Tprevious) to a second time interval (Tsampled);

continuing the refrigerant filling while determining subsequent changes in temperature (ΔTsubsequent) at a plurality of successive time intervals; and

comparing the ΔTsubsequent at each time interval against a predetermined temperature change target (Tthreshold) to determine whether to continue the refrigerant filling or to stop the refrigerant filling;

wherein the refrigerant filling is continued if at one or more of the successive time intervals ΔTsubsequent>Tthreshold and stopped when at one or more of the successive time intervals ΔTsubsequent<Tthreshold.