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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.
 

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Any evidence of coolant in the oil, or burning coolant in the exhaust? I’d probably do a compression test. That you replaced the cats and had misfires concerns me because if it was run with them plugged and misfiring long there’s a good chance of head gasket problems.
 

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Did you get the misfires only after replacing the plugs and coils? If the misfires came after, I would suspect something that was disturbed in the process. If you moved the fuse block , that would be a good place to start checking as there are two fuses, one odd and one even cylinders.
 

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Any evidence of coolant in the oil, or burning coolant in the exhaust? I’d probably do a compression test. That you replaced the cats and had misfires concerns me because if it was run with them plugged and misfiring long there’s a good chance of head gasket problems.
The O2 sensors were white when I took them out but no other evidence of a blown head. No coolant loss over the last 6 months.
 

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Possibly carbon buildup on the valves. Its a common disadvantage of Direct Injection engines.
 
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