O2 sensor question
 

If Im running a gutted cat will it take out the rear O2 sensor or did mine just fail from being old?

fail from being old

What do you mean by fail?

The rear O2 will see more heat on average without a cat element before it, and it will have a decreased lifespan as a result, but it won't fail immediately. Think of it as dying at 60 instead of dying at 80.

Without a cat, it's only remaining function is as a narrowband in steady cruise conditions (instead of using the slower responding front wideband), and as a check against the front O2's health.