Heater blowing cold air

[Wiley Blevins]

My heater is blowing cold air though I've already replaced my thermostat. My fluids are good too.
Any thoughts?

 

[Donystoy]

Check for hot heater pipes going into firewall. If pipes and hoses are not hot verify that the valve is not broken and stuck in the closed position. I just replaced mine where the actuator arm had broken away from the actual valve. Mine had broken in the open position but could just as easy break in the closed position. On mine the vacuum tube was also loose and had to tighten it by wrapping wire tightly around the end of the rubber tube fitting.

 

[Mbrooks420]

1. Low coolant.
2. Air bubble.
3. Plugged heater core
4. Blend door/control issues