p0432 main catalyst below effecineny help me
 

I have 2007 veracruz.the check engine light come out suddely. I brough it to the dealler he ckeck it it showed that p0432 main catalyst effeciency below thersold bank 2
so he told me you have to change the catalystic system
then he reset it then after a week it comes back again i check it with other dealler he told me dun change the system you have to change the sensor it self
so I'm confused now, what should I do change the system or the sensor
Any one can advice me