99 chevy silverado heater doesn't blow hot air, air blows but cold air only, both hoses under the hood going into firewall are warm, my fluids are full, maybe flap door is not opening, help, help, please... thank you..

CHECK TEMP. ACTUATOR MOTOR

In addition to faulty actuator motors, the blend door itself can also be broken. This happened on my Tahoe. Typically it is a very labor intensive and costly fix to replace a broken blend door, as the dash must be removed and the plenum assembly replaced. There is a DIY kit from heatertreater.net that provides steel hardware as well as instructions on how to perform the repair by removing the glove box rather than the entire dash.
This is also an alternative to replacing the 'difficult to access' top actuator motor underneath the dash. The kit contains a single blend door which converts the dual control to single control, and is operated off of the bottom actuator motor, which can be accessed by removing the glovebox. If you are having issues with your blend door, or the 'top blend door actuator motor behind the dash' it is worth looking into.

It could either two things the actuator control valve or the cliamate control itself check your wiring and fuses before buying anything