The system is sealed and self-purged, but does not mean that air remains inside that could not evacuate, especially if repairs were made where refrigerant was removed. Remove the cover of the reserve container, put the engine running, if there really is air, bubbles will be observed inside the container. Set the heating to the maximum for one minute (it is part of the system and should be considered as such) the engine will remain on during the procedure, therefore the interior temperature indicator will be observed.