You need to follow the same routine that a technician would to trouble shoot this before you start trying to change any parts. Is your engine reaching full temperature? Are both heater hoses too hot to hold to? From ther you really need a full scan tool to access the Heater/AC system. The temperature blend door is just about in the middle of the plenum assembly. The actuator for it is external and serviceable if it has a problem. To determine if there is a problem with the actuator you need to have this scanned.