Suzuki SX4, P0134/ P0420, sensor or CPU?

So, my friend is having car trouble with their Suzuki SX4, 2007.

I was told the car had some issue with the catalytic converter in the past. The mechanic told them it would cost $2000, but that it didn’t effect the vehicle performance aside from the amount of pollution in the exhaust. The owner opted to ignore the problem.

I hooked up an OBD and got codes P0134: Sensor circuit no activity detected bank 1 sensor 1.

and P0420: catalyst system efficiency below threshold bank 1.

Okay, so probably a problem with the O2 sensor, right?

The thing is, the symptoms don’t match the diagnosis. The problem was first noticed while they were driving. The internal lights flickered and the car stuttered a couple times, but they made it home.

A couple days later, they try to bring it to a mechanic but it doesn’t turn over. They jump it. It starts, but it sounds bad, and won’t shift out of park. When the gas is pressed it stutters and dies.

This doesn’t seem like it would be related to the O2 sensor to me. I assumed there was going to be some electrical issue. Is it possible there is actually some issue with the computer and it shows up as a sensor issue? Or is my intuition wrong and these symptoms be are caused by a shoddy O2 sensor?

Note that is has been unusually warm here, upper 90’s and humid for the past couple weeks.

I just want opinions before we drop the cash on a new O2 sensor.

I inspected the wiring. It looks fine at first glance. I haven’t removed the sensor yet to inspect it because things are stuck, and I’m lazy and want to hear back from here before putting in the effort.

Thanks for any input!