I've had the same fault code on and off this year. I've just been resetting it with my OP-Com periodically or ignoring it completely!
I've got more pressing issues with my car right now anyway, but on reading up on this code, I think it means the sensors before and after the cat are showing too similar a result. It can be caused by one of the sensors being knackered (or even a loose connection possibly), or it can be caused by a lack of efficiency in the cat itself (in which case you'd need a new cat).
If you have access to a oscilloscope, you can apparently read the voltages off and check that they are consistent with expected values, and makes sure all the connections are good to the sensors. I think the sensor on the engine side of the cat will tend to have a greater variation in its voltage, whereas the one after the cat, should be less fluctuating... which would show that the cat is doing its job.
I have only read about it, not done anything myself. My garage suspects I need a new cat, but said get everything else sorted first, and run it a while and see if the light is still coming back then and I could look into it at that point.
So I am budgeting for a new cat in my finances in case... but of course hoping the fault doesn't come back (so far so good!)
Mike