Heater not working properly

SOURCE: Intermittant hot/cold air from heater

Had the same problem with my Mazda6 awhile back. Took it to the dealer and they found the radiator wasn't holding pressure. They replaced the whole radiator under warranty (WHEW!). Not sure how to check for that kind of thing, but it's a start.

SOURCE: 04 Chevy Avalanche Driver's heater blowing cold all the time

your mode actuator is stuck. try unhooking battery for a few minutes when you hook back up actuators will do a relearn.if this does not work will need to have replaced

SOURCE: Problem with No Heat on 2005 Chevy Avalanche - Cooling System?

it is most likey a sticking thermostat it either gets stuck closed or open in your case closed so collant isnt getting to the heater core and all you get is cold air blowing into the truck.
its a simple fix you can either do it your self or take it to a mechanic. now remember there are differnt thermostes so think about the climat you live in and get one according to that

SOURCE: 1998 Chevy S10 pickup air condition compres is on instead of heat

First be sure that you do not have the controls set to defrost the windows since on just about all vehicles nowadays with the controls set that way the air conditioner will be turned on...even if it is not so warm outside. So...be sure that you do not have any of the controls set to defrost and that the heater is turned all the way on.

It sounds like though that the control for the vents is not operating properly. This is a combination of mechanical movements of the vent flaps and small "air motors" that operate the flaps. These air units are controlled by air vacuum taken from the engine compartment through tiny hoses. If the hoses get cracks, holes or come loose from their fittings then your vents will not move correctly so it is a matter of looking and tracing each one of the hoses to make sure they are fastened correctly, have not come loose and are not cracked or leaking. The problem can even be in the engine compartment where the hose(s) connect to the engine vacuum system...so it will take lots of looking carefully with flashlights etc....good luck.

SOURCE: My 2005 Avalanche Z71 has an intermittent heater

I had the same problem and took it to the dealer, come to find out, my antifreeze was very low, once it was topped up, the problem went away. There was no evidence of any leaks.