I am wanting to get a programmer but don't want the warranty to get voided if the onstar can tell its on there. Does the onstar detect an installed programmer?  

I think you have to sign up for the E-mail diagnostic service. It's not an automatic thing. I'm not 100% sure if the diagnostic can detect a re-flash. I'd probably lean more toward an in-line box, because it's essentially a signal modifier, rather than changing the base code of the ECU. Then it's just a matter of hiding the module so where it's not physically visible to technicians.  

Good question. I have an 06 duramax and registered for the online diagnostics. So far I have not received any notice through my emails of the flashpaq tunner I added. I think it only reads and notifies of any errors when something is not working right. Until then, I think onstar has no idea.  

My friend runs a programmer on his D-MAX. The only time his Onstar complains is when his Alison goes into limp mode because the programmer's set too high.  

I HAVE AN 08 D MAX WITH THE BANKS 6 GUN W/PDA AND ONSTAR CANT TELL ANYTHING IS THERE  

Yeah, I don't know if OnStar necessarily looks for that kind of stuff. You should be OK, although if you start getting error codes, those might show up.