Your heating coils are most likely clogged.
sounds like you have a bad blower motor.. i just replaced one on my 02 deville for the same reason with the same symptoms
Possibly the Blower Motor has stopped working. This was the case in my 1994 Acura Legend. I just replaced it, and now the heat works great again!
it's the blower motor
OK, did the blower motor actually just stop, or does it still blow but no heat?
The diverter, or blend, door, is not working properly, The door is either feeding the heat through the defroster, or closing the heat completely off.
is the blower motor running? or is it just not blowing hot air?
Could have just overheated
Does blower operate at all? If not it could be blower motor, fuse/fusible link, faulty switch, blower resistor pack...sorry not enough info for much more help than that (model, year, just heat not working...or a/c also?)
if its not blowing at all, your blower motor is bad. if its blowing but not cold, you probably just need a shot of coolant. tour blower motor is norm located under the dash on the passenger side, below the glove box.
The year, make and model info would help. Also need to know if the blower motor is working, just not getting any heat, or not changing function between defrost, dash or floor vents.
Some of the Rams had a recall on the blower motor wiring.I would check the wires at the blower first.If the wires are in good shape,then you may need to replace the blower.
Probably a bad thermostat.