Land Rover Repair: p1314 p1313, muffler shop, head gaskets

Question
I bought a 1997 range rover that needed some work(for a good price) the head gaskets were blown so I replaced them the check engine light was on. I purcased a scanner(actron 9175) and had several o2 sensor codes, so I replaced them. Now I only have two consistent codes left p1313 p1314.(which i think is both catalytic converters) What do they mean? How to fix it? How did it happen?
P.S
I thought that a bad catalyst would make the vehicle run poorly. Mine runs fine
Help

Answer
A clogged cat will cause poor running, an inefficient cat will only cause high emissions but not affect running.

the codes are
P1313-Misfire Catalyst Damage Fault Bank A
P1314-Misfire Catalyst Damage Fault Bank B

A good muffler shop should be able to determine if your cats are damaged/inefficient and replace with a universal unit for much cheaper than the LR replacement.