My 2003 f150 heater is blowing cold air, both heater hoses get hot and the temp gauge is in the normal zone [straight up]
Answer : The most common concern on these trucks is a broken temp blend door. It kinda hard to diagnose, If you were to have the a/c set to max air and on high blower and you turned the temp from cold to hot, you should here a change in how the air flow sounds while the door is moving. You really need to know what its suppose to sound like, if you know someone else with a working truck to compare to.
Make sure the knob is not broken on the temp,knob.. pull that off and look for cracks where is attaches to the shaft.
I suspect the broken door and it requires the whole dash be removed and the heater box cover removed to replace the door. Its a 8 hour job.