does it automatic air or manual, If its got manual put the fan on low then move the temp switch to hot and cold fast try it couple of times also check the little knob on the back to see if theres a crack on it if there replace it. If this works get a temp. switch, They are not expensive and easy to change. If yoy have automatic in the middle or on the side there is alittle hole about 1 inch long by 1/2 wide lighty blow air into it this will clean the dust out then it should work | 
