P015B seems 02 sensors, if you havent i would just replace them both, and P0171 is 02 sensor i think. I have a 2012 LTZ RS i fought the 171 code for a while. There is a purge sensor it has a hose connected to it. It is a hose that runs on top of the motor I replaced it and the code hasn't come back on. I'm not going to say this is your issue. I tend to do my own work. I trouble shoot by throwing parts at it and i still come up cheaper than the taking it to the Stealership. Thats the part number. Look into it. I might have had more going on it my car. Mine happened at the same mileage as yours 93,XXX. Hopefully this helps.
[h=1]PURGE CONTROL VALVE - GM (55577313)[/h]
[h=1]PURGE CONTROL VALVE - GM (55577313)[/h]