02 sti smelling fo fuel and now engine light on
 

the cars been smelling rich for a few weeks now on cold start etc also when stood behind the car with it running (seems realy rich )but today on my way to work i got the engine light come on
i have a scan gauge pluged in so i got the code of which is p0420

the car has a full milltek system on it which was put on before i bought the car and remaped with it and a few other parts done
so do i change the sensors? or is it more likely to be the cat ?

thanks