Heater Only Blows Hot Air

[shithappens]

Anybody have an idea why the vent will only blow hot air? No matter if it's on cold or a/c, vent or recirculate it only blows hot air. Although its cold out now this would be a real bummer come summer. The truck is a 2001 silverado.

 

Thanks guys.

[Draenor]

Turn off everything, disconnect both battery cables for a min or two. Reattach, and start back up. See if that helps.

[shithappens]

It's weird, I just used the truck again and now the heater works "normally". This is the second time it has done this though. Sometimes no matter what you do it blows hot air, then sometimes it works like it should. Ideas or suggestions welcomed.

[ZZ327]

its probably a damper door not working.