Could be,
Low on coolant,
A stuck open thermostat,
A plugged heater core,
A faulty temp blend door actuator,
blower motors bad check fuse bore replacing
it's old and you need to change it.
heater core plugged or air locked check to see if hoses going to heater core are hot
needs heater core replaced wost likely be hind the glove box
it is located on the right side of the passenger compartment of the vehicle you would have to remove the whole dash in order to replace it so if you have to ask where it is you probaly won't be able to replace it.
This would indicate that the system is low on coolant when the gauge is hot. Not enough coolant to service the heater core.
MaybeYou may have it on "auto" not off check all switches off You may have a faulty selector switch. Also some vehicles always blow heat even with the heater off.
Its your Heater core if it smells like a syrupy sweet smell if not then its your thermostat
To reset the computer on a 1998 Bravada you need to disconnect the negative cable from the battery. Leave off for 10 minutes and reattach.
in my experience it is a slight leak in heater core it will be antifreeze causing this replace heater core and problem will be fixed
if antifreez is blowing into cab, u need to replace the heater core the core has a hole in it. thses are not repairable and they are cheap to buy
YEs 4.3L