My heater used to work but now blows cool to cold air. I can tell the temp control switch is changing the flow and every other switch setting works as well. I have read about blend door issues and I don't think its my blend door. I have attemted flushing the core and actually had good heat after until I had to use the A/C again then back to cold...I flushed it a second time and could get some heat but it wouldn't last. From other articles I think I may have air in the system but not sure how to get that solved. Any help is appreciated.

 

Check your water level.

 

^^ this and have you tried disconnecting the heater core supply and return lines off the block and blowing them out on their own?