2010 PO138 code and flashing VSC light

Hello All. New to this forum and needing some help. I have a 2010 Corolla S with 142,000 miles I just got for my sister and a check engine light with code PO138 and VSC light (flashing) came on. Changed the downstream O2 sensor and reset codes and light came back on the next day after driving about 20 miles . Wondering if anyone knew what else to look for that could be causing this . Thanks all