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Could be,a stuck open thermostat,low on coolant,plugged heater core,faulty temp blend door actuator,
Remove (or have a professional remove) the center console. The cup holder may or may not be repairable.
u need a new high/low switch
Little or no heat from heater/defrost if t-stat is stuck open. Engine overheats if t-stat is stuck closed.
Thermostat stuck open, if older car possibly clogged heater core- when run at oper temp the hoses going into/out of heater core be sure one is very hot - start with thermostat $$$ cheap & easier than a heater core problem!
Check for a vacuum leak either under the hood or under dash Check the operation of the vacuum actuated motors under the dash Check the operation of the selector switch in dash Defrost is the default position for this system
If it's stuck in defrost your battery died and messed up the computer. Disconnect battery for 5 or more min. It worked on my 96
Check behind the heat control panel to be sure that the vacuum hoses are all securely plugged into the control switch. As a safety measure on all cars, if you lose vacuum altogether, the default position for air direction is on defrost, so you can always see out of the windshield. Jake McCahn
Pull the blower motor out and check both the motor, the blower wheel and the housing for any trash that may be stuck in it................
The air door under the Blazers dash could be stuck, or there could be a vacuum leak on the engine. The door opens and closes from vacuum, directing the air from heat to defrost.
The fan will run continuously if the A/C is on or the heater is set to Defrost. If that is not the case then either the thermal relay for the fan is stuck closed or the engine is running hot.
It sounds like your air flow diverter is stuck on defrost. Basically it is a little door that diverts the air fom vent to defrost to feet. It could be either the heater control valve, or te temperature selecter cable is broke.