Broken Heating System on 2006 Mitsubishi Lancer

The heat in my 2006 Lancer oz rally stopped working, but air still blows out even when I have the heat on but it blows cold air out. Iv'e taken it to various places to have it inspected but no one can seem to point out a cause. Any idea what might be the problem?

Asked by for the 2006 Mitsubishi Lancer
Make sure coolant level is full first. Then possible check thermostat.