The most likely cause is a failed blend door actuator; however I recommend a little troubleshooting. First, can you hear a clicking sound behind the dash when you turn the setting to hot? If yes, it is certainly the actuator stripping its gears. If not, check to make sure the heater hoses are both hot as they connect to the heater core at the firewall (in the engine compartment). If one or both of the hoses do not get hot, there is a blockage caused either by an air bubble or a stuck water control valve in the heater hose. If the hoses are both hot, the indication is again the blend door actuator.
Below are instructions for replacing the actuator. Look at the diagrams carefully--the big arrows point to the front of the truck and the actuator is on the side of the heater box above the outlet duct that runs to the driver's side of the floor. Also, the actuator is expensive, so I recommend you make sure it's bad and shop around a bit to get a good price. To check out the actuator, first see if you can turn the blend door by hand. If you can't turn it, maybe it's jammed. Also, reconnect the actuator after removing it and see what it does when you turn the heater to hot.
Removal & Installation
- Remove the floor air outlet duct extension (1) from the floor duct (2).
Fig. Remove the floor air outlet duct extension (1) from the floor duct (2)
Disconnect the electrical connection at the blend door actuator (#10 in pic below).
Fig. Blend door actuator and related components
Remove the blend door actuator mounting screws from the HVAC module.
Remove the blend door actuator as an assembly with the actuator cam.
Fig. Remove the blend door actuator mounting screws from the HVAC module
- Install the blend door actuator as an assembly with the actuator cam.
- Line up the heater/defroster valve lever with the blend door actuator cam slot.
- Line up the A/C valve lever with the blend door actuator cam
- Install the blend door actuator mounting screw to the HVAC module.
- Tighten the blend door actuator mounting screw to 14 inch lbs. (1.6 Nm).
- Connect the electrical connection at the blend door actuator (10).
Install the floor air outlet duct extension (1) to the floor air duct (2).
Picture of actuator: