91 corsica heater problem

My son has a 1991 Corsica and the heater just stopped blowing warm air.
It blows good so the fan is working, just blows cold air.
You can turn the temperature adjust knob and nothing happens.
Any ideas on what the problem is and some helpful hints on how to correct the problem would be greatly appreciated.

Thank you

Try checking the coolant level to make sure it's topped off.

Check the thermostat first. I doubt it's the heater core unless it has been leaking coolant inside the car.

My son has a 1991 Corsica and the heater just stopped blowing warm air.
It blows good so the fan is working, just blows cold air.
You can turn the temperature adjust knob and nothing happens.
Any ideas on what the problem is and some helpful hints on how to correct the problem would be greatly appreciated.

Thank you
Sounds like the heater core is plugged up. You can try and back flush it, but I did that and wound up replacing it.