heater is blowing cold air on idle or downhill... on 2003 Honda Civic

coolant levels are fine..I have replaced the themastat...that didn't help...bought a new radiator cap... that didn't help..the car has never had an over heating problem AND the carbon levels are fine on the smog test so there is no problem with the gasket...humm...can I check the heater valve lever from the inside on this car? Should i check that first before paying to have the heater core checked out? thanks!!

Asked by for the 2003 Honda Civic
Yes, I would do a flow test of your heater core, it may be clogged. Can you hear the blend door move from cold to hot when you change the temp setting, because a non-functioning blend door can cause this as well. I don't see a heater control valve listed, so I would suspect that you have a blend door system.