P0138 & check engine after cat converter replacement.

Long story short, at some point last month I got my catalytic converter stolen. I got it replaced at a local mechanic because Toyota wanted nearly 3000€ for the replacement. Anyway, a few days after the replacement, I got the check engine light. Checked the codes with a odb2 connector and it says it's P0138 (O2 Sensor Circuit High Voltage). Went back to the mechanic and he said that the sensor got damaged during the theft, and that water may have gotten inside the sensor or something like that (don't remember), and that I shouldn't worry too much and save the money. Car runs just fine and mileage seems normal to me, and sometimes the check engine light turns off on its own, but it usually comes back after a day or so.

That being said, I don't believe this should be ignored. What do the true Prius experts think?

Also, wouldn't running with the check engine light permanently on hide other problems that would make the light turn on, read: dangerous?

 

Hello again everyone. All of a sudden the check engine light turned off and has been off for several days now. In the meantime, I bought another odb2 device and there doesn't seem to be any error right now. What do you think?

 

After the light disappeared for a few months, it came back with P0420. I think either the sensor is actually damage or the whole cat is on its way out already. Maybe he scammed me? Car still runs ok I guess, but I'm getting a bit paranoid, also it's a bit annoying to drive with that yellow light constantly looking at me.