If the heat seems to come and go, this sounds more like a heater core issue than the blend door.
However, if the entire system is not operating then that sounds like a control issue.
Here is a guide that will help with this issue:
Let's clarify if the front and rear heat are coming and going at the same time or not.
I would suggest checking the coolant level and flushing it if it looks like because you could have an air pocket if this is just the front or rear system.
The next thing I would suggest is let's check the temperature of the heater core lines and make sure they are the approximate same temperature when this issue is happening and that will tell us if the heater core is restricted.
Let's start with this and go from there.
Monday, April 11th, 2022 AT 6:09 AM