No heat when fan on 4 or 5

[yukontario]

Hello Folks, new on here. I'm sure i have duplicated this question but dont seem to find an answer. I have a 2001 Yukon xl slt, rear heat works fine. Front heat worked in the morning, stopped for 2 hours and the heat blows cool on fan speed 4 or 5 and blows warm on 1 thru 3. It seems to get hotter on 1 thru 3 when driving but not on 4 or 5. Any thoughts? Thanks in advance.

 

[vwbeaner]

Could be hvac controls aspirator in ceiling, blower motor resister. Not sure just a few things to look into