2009 Buick Enclave CXL AWD with 98k. I have a noticeable misfire with codes reading misfire on cylinders 1,3 and 5. All coils have been replaced and all plugs are new. I also get a code reading catalyst system below effective threshold. I replaced the O2 sensor on the back side, pre catalytic converter, and still get the misfire codes. No vacuum leaks as far as I can see and detect. Do I need to also replace both O2 sensor in pairs on the back side? Assuming 1-3-5 are considered the back side.