Hey there, just looking for some advice or anecdotes around this: check engine came on during a pretty long drive (3 hours). Stopped at an autozone to use their code sniffer gizmo and it came back with a P0421. Googled the code and it looks like it's either the O2 sensor(s) or the catalytic converter(s?). Looked around on rock auto and it looks like if I wanted to replace each of them, it would cost around $400 in parts. But then I started reading about the CC system and it seems like there are two modules: one on the read of the engine and the other underneath. That one on the engine seems like it would be a massive pita to fix with limited tools and parked on the road. The O2 sensors don't look too bad, nor the CC under the car.
But most of my stuff is out of state - I've been moving for the better part of 8 months.
So my questions are: has anyone attempted any or all of these replacements that could give me a pita score from 1 to 5. OR does anyone know what I can expect to spend for diagnostic and repair, given the code?

Thanks!
2014 CX-5 | 2.5L | AWD

 

I would highly recommend that you just have a shop confirm what that part(s) that are failing or failed.
If you start assuming by looking at your HO2 that they are good or bad and replace them without confirming first if they are the problem, you can add to what is already a problem.
Your CAT can become damaged by guessing at other parts and replacing them. Your CAT is very expensive why contribute to causing it to fail. Also if the CAT fails due to the wrong diagnosis other emission systems and parts can be effected and damaged. That DTC code is not one that should be taken lightly.
If you go to check your monitors you will see several that are NOT IN READINESS and effect several emission related parts.

It would seem you are a DIY and capable of part replacement so why not play it safe pay for an hour of shop time and get exactly which part is needed replacing or repair?

 

Callisto nailed it, make sure you get someone who knows what they are doing to look at it first and determine what caused the failure .That will be the most cost effective way to deal with it. If you end up replacing the cats , get good OEM quality cats.