Low on coolant.
Plugged heater core.
Temp blend door inop.
if it doesn't heat your heater core is most likely clogged and if it doesn't blow at all then the blower is messed up.
Have you checked your suction lines.
why doesnt my heater work on my1985 BMW
When you have the heater on, does it just blow any air at all? or no air?
Check the following items.................... 1. A/C heater controller. 2. A/C heater controller doors. 3. Check for vacuum leaks and cracked lines.
get a new heater switch
The heater control valve is stuck open.
Low coolant in the system. Defective heater control valve. Clogged heater core.
check heater fan to see if it is working.
Check your anti-freeze levels
Likely heater core. I had same problem with 1996 S-10 with 4.3L.
Low on coolant. Stuck open thermostat. Plugged heater core. Faulty temp blend door.
May need to replace blower motor.
The heater valve could be failing to open for one reason or another.
Replace the high speed relay.