Possible Causes

WARNING: This page is about a different car, the 2004 Mercury Mountaineer and 2004 Ford Explorer. However, it is still accessible from the selected car via links, so may be relevant.

(DATC systems only) An open, short to voltage, ground in circuit 1069 (OG/LB).
(DATC systems only) ATC solenoid and manifold.
(DATC systems only) DATC module.
(Manual climate control systems only) Function selector switch.
Heater control valve.
Heater control valve vacuum switch (if equipped).
Stuck or bound linkage or door.
Leaking or plugged vacuum hose.
Temperature blend door actuator.