if the o2 sencor is bad, would it throw a code? is it possible for it to be bad and not throw a code? thanks

 

it should, and I have herd of people having bad O2s with out throwing codes.

 

FWIW, my O2 sensor was was slow and lazy, and had low output. It did not set a code because the ECU looks for activity from the sensor. If it doesn't see ANY over a period of time, it will code. Low and slow output keeps the ECU happy, but the system isn't operating as it should.