Does anyone know some of the symptoms of a bad o2 sensor?

 

poor gas milage, bad idle, startup, might even smell gas in exhaust " rich exhaust"

when you replace your O2 remember to pull the battery so that the computer resets so the car can relearn the sensor data.

 

Thanks. I bought a OBDII scanner and the code came up as the 02 Sensor.

 

What was the exact code? P420 is for a bad cat which is known on the 6th gen Monte's. I hear also of questions if this is bad O2 or not and it is just a bad cat.

 

Ok now my gas mileage has become about 2 to 3mpgworse and the exhaust smells rich. But I am not shooting up a code/no service engine light. Now I do have a little over 100k now and I know the O2 sensors are said to be good for 100k but not having a service engine light on makes me wonder if it is an O2 sensor. What do you guys thinks?