Last year my catalytic converter was stolen in my 2008 Mitsubishi Outlander. It's the 2.4L 4 cylinder model. In the same week I went to a shop and had a universal cat put on. I didn't think much of it at the time because I didn't know much about catalytic converters, I just wanted the loud exhaust under control.

Since then I have a check engine light and after scanning for codes I have a few:

p0131

p0134

p0137

p2195

When I looked them up they mostly seem related to the oxygen sensors. My car only has one upstream and one downstream. I had replaced the downstream sensor when I replaced the cat. I've talked to various mechanics and some have told me to replace the sensor and others have told me to replace the cheap cat. I'm currently stumped at what I should do. The cat was a generic universal one and the sensor was from DENSO.

Is it possible that the cat is not working for my vehicle and I should replace it with a better brand? Or should I replace the O2 sensor for something else? Both are expensive parts and I'd just want some advice on what I should try first.