I currently own a 2007 Chevy HHR LS. A couple months ago, my check engine light came on and the problem was one of my O2 sensors. After about a week of being on the light disappered before I could fix the problem. Now my car's gas milage is terrible. Im wondering which O2 sensor(s) do I replace to eliminate this problem. Thanks

 

The code will still be stored in the ecm. get the code read and you will have a starting point.