Can a o2 sensor be bad and still not throw a code?

 

Possibly but not likely.

 

If its not getting any reading it will eventually throw a code. Or if something off and giving an improper reading then a code will throw

 

I haven't seen a bad O2 sensor that didn't throw a code. I guess the real way to check it is to track your MPG. If an O2 sensor went bad you'll see a drastic decrease in fuel economy, at least that's what happened when mine went bad.

 

^^same, had one go bad, car was furious, drank gas like a truck. almost always gonna get a code for a bad o2 sensor.