The first thing I would do is smoke test the intake system for leaks. On older cars, that's just something you should do first if the car engine is not running right. I found a couple of leaks on our 2014 RRS, and the main leak causing misfires was an leak you could not see in the symposer hose, so I blocked that off.

 

Air leak -> lean, not rich.

Bad/dirty MAF if both banks otherwise maybe a bad cat conv.

 

How does a bad cat converter cause a rich condition? please explain

 

Any/all of: ruins the exhaust gas flow, messes with back pressure, confuses O2 sensors.

Proper engine running relies on all those (& others).