Hate when businesses have attitudes like that. We don't have either of those stores in Canada but similar ones. Haven't really had a bad experience with any of them, and with me being a licenced automotive technician I get a pretty good discount. Anything my family owns is GM, so I usually get parts from the dealer I worked at for 7 years, they're still good to me and give me my employee discount lol. I'm lucky to have a Tech 2, which is the GM scanner. Funny you mention that Mudrunner, my girlfriend's Trailblazer also flagged an O2 sensor code about a month ago, still haven't got around to changing it!
William, wouldn't hurt to buy one of the cheap code readers like you were saying, then you have it. You can get a decent one for under 100 bucks. I had one from Matco that was about 150, it even read live data which was handy. Had it in my toolbox in case I was working on something other than a GM. Worked great til someone liked it more than I did and stole it.