Hello. My Titan wont switch from defrost to ac vents sometimes.

you have a vacuum leak.more than likely under the hood,next to firewall,the smaller vacuum lines most likely,but check all of them.if no cracked,split,or dry rotted lines found there,then look under the dash at controls.when you have a vacuum leak the blower motor will default to the defrost mode.listen for hissing sounds.check all vacuum hoses.