The person I bought my Trailblazer from removed the check engine light without telling me but I had a feeling it was on after a while so I went to AutoZone to check it and it threw code p0172 P0171 P0340. I have real bad vibrations and bad idle and really bad fuel economy. I'm thinking of replacing both O2 sensors and the camshaft position sensor. Is there anything I should know before doing this? I'm getting the cheaper parts on rock auto. Or could it be something else?