I have a 2006 GMC Yukon XL with a dual zone ac/havc unit. The rear heat works but the front only blows cold air. I have checked all relevant fuses, checked the climate control module, and all are good

I finally figured it out on my own. Apparently there are two separate heater cores, a front and a rear. This I did not know until this morning when I went to flush the core. The front core was plugged with residue and the rear was eventually getting to the same point. I flushed both cores and I am producing good heat both front and rear.