heat problem

Alero 99, 3.4L the fan works properly, but when temperature knob in red area-heater does,t work and air is cold. Any idea ????
Regards

The first thing to check is that your car's coolant level is full. A low coolant level can cause the heat not to work.

The next thing I can think of is that your thermostat may be stuck open, not allowing the engine to get up to full operating temperature. Your temp guage should be at or near the middle of the arc once the car has been driven for a few minutes. If not, then it could be an indication of a thermostat stuck open.

The next thing possible, is that your heater core could have an internal blockage. You could have the cooling system flushed and see if that helps. If it's got serious blockage, even flushing may not help.

The last thing is that the mode valve that's operated when you rotate the switch from blue to red or vice versa, may not be working properly and is not moving the door that directs the air to go across the A/C evaporator or the heater core depending on where you have the knob. You don't even want to think about what it takes to replace this part, so hope it's one of the other things.