A/C is locked only on the defrost

I started out hearing a clicking noise when I turned the heater on, I took it in to be checked, asking for the noise to be checked out and find out why the controls wasn't working. the air now runs only on defrost and the mechanic said the entire heat/air controls needs to be replaced... I only have 77000 miles on this vechile, this should be covered under warrenty or be a factory defect. I have own may vehiles and none have done this. I didn't have the vin number at time I posted this... but will