Code P0138

What is your question?

Hi, I just bought a used 2002 Chevy Cavalier yesterday and this morning the check engine light came on, on my way to work this morning. I went back home because I have a hand held diagnostic tester. The code is: P0138 o2 sensor circuit High Voltage (Bank1 Sensor2). My questions are, is it serious. Do I need to fix it promptly? Is it something I can fix myself? I don't normally work on cars, but I do have a 48 cc motor on a mountain bike, and I've fixed issues on that before, & I'm good at working with my hands, and don't mind getting them dirty :) Thanks! Joe