P0138 w/ brand new catalytic converter

My 2010 Aveo with 117k miles on it has been all over the place this summer. Had a persistant P0420 code, wasn't going to pay for a new catalytic converter (had a misfire in January, so was pretty sure it was a failed converter causing the code) but also couldn't afford a new car just yet. Through the generosity of some friends, I got a new catalytic converter put on my car from a really reputable repair shop. Before they replaced the converter, they checked both of the O2 sensors for their functioning. About 24 hours after picking my car up with the converter, my OBD pulled up a check engine light reading P0138. I tried clearing the code, but it came back. The P0138 code has been consistent now for 2 weeks.

Is this definitely a failed O2 sensor? What else could cause this? The RPM have also been really slow to reduce to idle speed after starting the car cold (when the engine is warm this problem doesn't happen). Was thinking it could possibly be a clogged fuel filter? Any thoughts?? I'm moving cross-country from Chicago to Colorado in about a month and a half and am nervous about the car being safe to drive into the mountains :shock:

I may have just been the o2 sensor in the first place. A bad cat does one of two things:

1. It clogs, leaving you with a "pressure" problem. I have seen completely clogged cats lock up engines, blow out exhaust gaskets, or o2 sensors from the manifold. Best case scenario, you notice the power loss and the secondary o2 throws a code. Worst case, tow truck and new engine. Anywhere in between is sort of a diagnosis between o2 and cat.

2. It blows out. When this happens you will not notice a power loss. It might sound louder, the exhaust will possibly smell worse and the o2 will still throw the same code. This one is tough, because it has to clog to blow out. Its really just the safer final result to the engine.

Hmmm... have you seen o2 sensors fail, but still seem to give readings when tested? My auto shop checked the o2 sensors before replacing the cat and showed me graphs of the o2 sensor function (before cat replacement and after cat replacement) and it seemed like the o2 sensors were working! It was just 24 hours later that my car pulled up a P0138 though...

Planning on replacing the rear o2 sensor and possibly the fuel filter too depending on how easy it is to replace (having a hard time researching it?!).

To my knowledge, and I researched the crap out of this, although there is a poster in this forum who claims to have found where his 2010 chevy aveo fuel filter is with pictures, it has been my experience that the fuel filter is in the fuel tank and not on the bottom where he found his. Not sure why or how that happens but My car is 2010 Chevy Aveo LT and it definitely does not have a serviceability fuel filter. I guess I could drop the tank and see if I can get to it but I'm betting against it. Even though I did find the same filter for sale, many retailers refused me telling me that it was not serviceable.