I asked this question a while back....as a prior to getting a jap-spec cat.......
the sensor is supposed to detect if there is a blockage in the cat - such as excessive carbon build up in the cat pores - which would stop it flowing and cause a build up of heat.
It such situations (i.e. - when the cat light comes on) the computer goes into a limp mode to help you get to the next garage.
if your cat light is coming on, it's probably either because your cat is rooted, or because welding the sensor has fuXXed it.
I suggest you look into it either way, as your car will be running well below par.