Based on my recent experience with O2 sensors, it is a matter of locating the bad sensor, and then locating a replacement sensor. My struggle was getting the old one out. Like a couple of the people in YouTube videos, I had to use a propane torch to heat the base so I could loosen the sensor enough to get it unscrewed; in the end, I did not use the special O2 sensor socket, only a 7/8" open-ended wrench. The mechanics wanted way too much to do the job I could do in a little more time.
So, the simple answer is, you can do it yourself barring any extreme access issues. Watch a few different YouTube how-to videos, buy the best sensor (OEM), and get to work.
Butch and Peggy
N.Central TX; 2000 Itasca Suncruiser 32V; Workhorse P-32 Chassis; Chevy Vortec 7400 L21 w/Banks Pwr Sys; Steer Safe; Springs/Airbags front; Timbren Suspension kit rear; Onan 5K