I put in brand new O2 sensors and the computer threw a check engine light saying bad O2s. In the city, I average about 12-13 mpg according to the GM computer, but on the highway I get between 21-22 going 60-70 mph. My highway mpg went up after the install, and my city mpg actually declined.

Why do I get a code with brand new ones (I didn't have one before I replaced them)? Would that cause only city driving to suffer, but not highway?

PS. I got the sensors off ebay from Global Automotive who seems to deal solely in automotive sensors. It listed them as OEM Delco O2s. I guess this might be why they were $30 instead of $100+

Thanks for any thoughts