faults like this are incredibly difficult to diagnose without really good bidirectional scan tools
Agree. You could check the AC (leaks) but doubt (re you just had it checked/gassed) that's the REAL problem. My suggestion is wait... See if it's the same 3 weeks from now.
As Fu kind of suggested, it's likely the H/U install. TBH, I'm not sure many of these H/U fittings are CORRECTLY installed to align FULLY with the GM VE specs. Mine was supposed to be fully compliant, but alas, it was MOSTLY compliant. When I had the Alpine kit installed (H/U with HVAC) JUST DIDN'T WORK properly for about 3 weeks. Now, without doing anything at all - it works, most of the time.
I'm not suggesting it's all 'black magic' - what I am referring to is the HVAC on these cars is highly temperamental. The problem is that while there is absolute science behind what actuates, and when, there seems to be a hell-of-a-lot going on, and therefore what can go wrong. Also, these are VE's, so the chips were simpler... Just one item sending back the wrong response can cause a multitude of problems, that, might, eventuate to completely unrelated items flagging an 'error' (because, the item does actually receive an error! - it's computer programming 101 - a chain reaction back up the line).
This is why many people on this forum start with the basics (akin to, "have you turned the computer on at the power outlet").
It's also a major reason why commercial pilots MUST go through a checklist when a fault occurs (drastic or otherwise) I digress...
Sorry that this doesn't help immediately or directly - and please forgive me if I sound arrogant - because I genuinely don't mean to be - I just see volumes of posts with 'no cooling', 'no heating', 'no fan', 'lack lustre' air, 'wrong vents' on the forum and many (not all mind you) are resolved by following a disciplined and discrete PROCESS of a checklist.
I wonder if any guru here could write up something that could be iterated over by a few others and perfected over time. It might help many.