why does my heater blow cold air

A couple of things, is the temperature gage running in the normal range? No change from where it was? If so, thermostat would need to be replaced. Then there is the AC actuator module up under the dash that may be bad, It controls the vent system in the dash, part of it being the outside air blend door (flap, whatever) that controls the temperature inside the cabin. Other than that, may need a coolant flush as radiator and heater core may be plugged a bit.

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I don't no what to do

I have the same problem with my '06 Silverado which has dual temperature zones. It only blows cold air intermittently and only on one side. Any ideas?

Check all radiator clamps and make sure (replace) they are holding pressure and are not rusted causing pressure loss. factory clamps are spring steel prone to rust fatigue. I have found this and it causes circulation problems. eg - coolant not going through heater core.

on my 2006 chevy truck i found air trapped in the top of the heater core,,, i took the hose off and filled the heater core up and it works great now on them trucks the over flow bottle is lower then the top of heater core,, this will fix it