Stupid Codes..02 sensors?

[ch229151]

Hey guys,

Just pulled a couple of codes from my 3.0 A4...

P 421
p 431

I assume since they both read the same error this has to be my post and pre O2 sensors? Is it easy to change them myself? Just wanted to check with the general concensus to see if this what it means before i go and order these things online....Thanks.

PS- I am getting crappy gas mileage.

[FourRings80]

Did you get the oxygen sensors resistance.....should be 2.5~10 ohms

Cats are covered under warranty right? (8years/80000miles)