Hi Scotty, I drive a 2005 Scion Tc with ~130,000 miles on it. Recently I got a P0420 (Bank 1) code come up so I decided to check out what might be causing it. My catalytic converter seems to be running fine based off a temperature and backpressure test. I also didn't find any exhaust leaks, so I started checking out my O2 sensors. These sensors had never been replaced and the upstream sensor was looking worn, but no signs of oil or soot, just normal wear. I tested its voltage response and it fluctuated between 0.1-0.6 volts. The downstream sensor however had a black residue near the base of the sensing area. I have not tested this sensor yet.

Sparkplugs were recently replaced at 125,000 miles and were looking very worn when I pulled them out during the change, but no sign of burning oil, or abnormal wear, other than just being old. I wanted to clean up any built up carbon in my combustion chambers so I used a Royal Purple fuel treatment to help clean things up about a month ago. I've also switched from cheap gas to using "top-tier" gas for its additives. Could these additives have loosened up any gunk in the catalytic converter that would lead to the downstream sensor getting dirty/failing? Could this downstream sensor be what's tripping my P0420 code, what about the upstream sensor?

Thanks for your help!