air conditioning blows great and cold out of two passenger side vents on 2000 Buick LeSabre

air blows nice and cold out of the two passenger vents , but on the drivers side it blows warm air. still has the same force as out of the passenger. all vents are controlled by sliding the lever open or closed on the individual vent. anti-freeze is full. I was told there could be a vent door motor ? not working ? not sure what to look for first. thanks for any help

Asked by for the 2000 Buick LeSabre
Low on refrigerant , needs re-charged.