high-flow cat/ race pipe?

[herostar]

I was wondering if taking the catylytic converter off of my car and putting on a test (race) pipe would make my 02 sensor not work, or if they are even related at all...

 

[L2RTSiAWD]

Only your second O2 sensor would freak-out but mines has never tripped the CEL.

 

[herostar]

my second o2 sensor is bad right now and I'm getting a new one put in w/ my cat-back next week... right now i get like 18 mpg and the check engine light is always on, will this start to happen again once the cat is gone? also, what does it mean to trip the CEL? thanks L2R