The light is for the lambda sensor after/in the cat.

The fault code is to tell you CO2 below threshold or something like that, to say the cat is not working correctly (not sure what the actual code number is)

The car will run fine with it on, don't worry about that. As for turning it off not a clue mate. I use the DSci Platform so when mine comes on (Focus ST) I just clear the fault code with my laptop and it doesnt come on again for a while. Bit of a ball ache having to do it via laptop/odb quite often but thats what I get for fitting a decat.