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check you antifreze level if it is low you wont have eat or if you get lots of fog your heater core is bad
it wont blow heat itll blow cold air
check heater fan to see if it is working.
Your heater core is likely bad
Likely heater core. I had same problem with 1996 S-10 with 4.3L.
Low coolant? Thermostat not opening? Plugged or restricted heater core? Heater core airbound? Heater valve control cable detached or broken?
somtimes a mouse moves into the blower motor and makes a nest ?
it could be that you've put too much coolant to percentage of water in
if this system uses dex-cool flush the system
try the bend door
either a bad fan switch or a bad blower motor. Take it to a trusted mechanic.
What temp. thermostat did you put in? Try a higher temp. one. or check to see if heater core is clogged