Find where the heater hoses actually pass into the car. They are usually found going into the car behind the engine at the firewall. There should be two black rubber hoses, about 5/8" diameter. With the engine running at operating temperature and the heater on, both hoses should feel pretty hot. If one hose is hot and the other feels cold or lukewarm, the heater core is most likely plugged.
This will tell you if heater core is working right. Then if good it has to be in HVAC controls. The temp blend door is first suspect. By locating and watching if it operates while dialing heat will tell you if controls or actuator itself is at fault. See pic.
Monday, January 2nd, 2012 AT 7:54 PM