Is my o2 sensor bad or cat bad, here's the reading?

Just to clarify this is po420 not 240 right? The front 02 sensor should be fluctuating, that's normal. The rear should be steady when rpm is steady if your cat is good. If this is a 4 cylinder you only have 1 bank. If your rear o2 sensor (post cat) is not reading steady at a steady rpm then you may have a bad cat or the sensor is bad. You shouldn't have any change in fuel economy as that o2 doesn't control the fuel mixture. There are some electrical tests that can be done to determine if it's the cat or sensor but if you don't normally work on these types of systems you may be better off bringing it to a shop for a diagnosis. You can also tap on that cat lightly with a mallet when it's cold to see if the material inside has broken apart. That would be a good sign the issue is the cat. Use this link to find a good shop near you. https://repairpal.com/repair-shops Alex