code 21 help please

[LegionGS]

Hi guys, I recently got a check engine light. I did the key dance and I got the code 21 that means '' Upstream H02S stays at center '' I just got my car running on streets again.
I recently repair this things:
- ECU (brand new)
- crankshaft and camshaft sensors new
- changed my cat-back exhaust to the stock one.

my question is that can if I should take it to autozone to remove that code? or it has to be something wrong when I change the stock cat back on my car? and btw the o2 sensors where recently changed brand new so I don't think that they went bad. What you guys think? any had a similar problem like mine?

 

[Dsmkauai]

So the code is for the front o2 heater. This could mean a bad wire from o2 to ecu, or it could mean the o2(heater part) is dead, so I would look into which and fix it. You can clear the code all you want, but if the o2 is really malfunctioning it will just come back.