I have a 2010 cobalt with 2.2 and 5 speed. It has 129,000 miles on it. I replaced the O2 sensor because it said high resistance. I replaced it with a denso O2 sensor, then a bosch with the same result

I forgot the code is P0030 for the car.

It might not be the O2 sensor http://helpforcars.net/obd_codes/p0030.html

Have you reset the ECM to clear the code?And if you have,and code is still there,check the wiring going to the 02,s the may be bad.

I get Bad O2 before the cat (You have 2 sensors ,,, 1 before and 1 at the discharge side of the cat. Read this explanation in everyday terns ~~~Testing O2 sensors that are installed ~~~ http://mr2.com/TEXT/O2_Sensor.html .. Good Luck