If it's the Warm Up Cat Efficiency code (P0421) Then ignore it. Just clear it and see if it re-occurs.
Adding an O2 sensor extension will cause a guaranteed P2096.
If it is a P2096 already, Post CAT AFR mix too Lean, then your O2 sensor might need a clean, or the sensor might need to be moved a bit further into the exhaust flow, not out of it.
Brad, Troy did extend it past the cat already. It needs a spacer, the dude at Mazda told me that as he had the same problem in the race MPS. Brackaus, thats not a bad fix, plan B
I think the O2 sensor needs to be cleaned.
It has taken 4+ months for the CEL to appear. (With no extra mods?)
To much unburnt fuel will cause the O2 to become dirty.