P0420 problem

Hi all, 

I need help with my ford, any advice would be really appreciated. A few months after I bought my mk6 the engine light came on, so I looked up online and invested in a OBD scanner. Connected it to the car and it produced P0420 catalyst system efficiency below threshold bank 1. After hours of searching up the code and doing research I come to the conclusion that it was either the lambda sensor or catalytic converter. So thought I'd start with the easiest (and cheapest) option and changed the lambda sensor (the one behind the engine). Connected the code reader again and wiped the codes then went for a drive, the car felt a bit healthier all round and run perfectly, the old lambda we took out was black so thought changing it had solved the problem. Or so I thought... 5 weeks later the all too familiar orange light comes on again, and once again producing the same code. From what ive read online other people have had the same problem, but some saying even changing the cat doesn't help. Is there anything else I should look into? Or anything that causes a cat to pack up? 

Thanks in advance 😊