Cold air blowing instead if heat on 1994 Chevrolet Suburban 1500

I've replace my thremostat and my heater is still blowing out cold air. What could be the prolem?

Asked by for the 1994 Chevrolet Suburban 1500
Most common problem is low coolant.
2 answers
This could be caused by low coolant level, air trapped in the cooling system, or a plugged/clogged heater core.