I think one or both of my cats is bad. I have been looking for the evidence before I shell out the $$ to replace it/them. Disco 2, 170k miles. Bad on power, Grand-caravan-has-more-giddy-up kind of bad. Plugged in the OBD monitor and found Cat Efficiency for both banks. Cat eff bank 1 = 115.54, and Cat eff bank 2 = 59.76

I imagine they should in a perfect car be equal. Which one is bad? Would you just replace both?

 

See

http://jeffreyfall.com/landrover

 

I straight piped mine, no limp mode, but it hemorrhaged fuel.

 

If you aren't getting codes set, I wouldn't worry about the numbers. The two cats aren't exactly the same.

If you are getting codes, then replace whichever cat the code is for.

 

For a P0420 cat code, before replacing the cat it may be worth your while to do an induction cleaning once or twice to see if that mends the cat.
I used to work for a shop that did the BG services and 50% of the P0420 codes were resolved from doing a proper induction cleaning. The cleaners made their way into the cat(s) and got rid of the crap that was causing the efficiency issues.