A few months ago I saw a loose wire hanging underneath and took it to my mechanic and he said the O2 sensor was disconnected and I needed a new one. What would change performance wise if I get a new one as I am driving without one now.

 

without the O2 sensor, the fuel injection is running rich. you'll get better fuel economy with it working. I'm rather surprised the 'check engine' light isn't lit, there almost certainly should be failure codes that can be read with the under-hood diagnostic module (blinking LED)

 

I think it depends on which one is bad. If it's pre cat it matters, if post cat maybe not.

 

did they ever put dual O2 sensors on a B230F(T) ?!? Not that I've seen anyways.

 

Thanks guys and yes my check engine light is on.