1993 Chevy Truck heating and cooling

Heater Core "” Plugged

WARNING: The heater core inlet hose will become too hot to handle if the system is working correctly.

Check to see that the engine coolant is at the correct level.
Start the engine and turn on the heater.
When the engine coolant reaches operating temperature, feel the heater core inlet and outlet hose to see if they are hot.
If the inlet hose is not hot:

the thermostat is not working correctly.
If the outlet hose is not hot:

the heater core may have an air pocket.
The heater core may be restricted or plugged.
If both are hot, the temperature blend door in the heater plenum may be broken or stuck.
You may also have a shut off valve thats operated either electrically or by vacuum, and it may be stuck closed, this valve would be on the inlet line to the heater core. It's called the heat control valve.