ford focus fault code P0420
 

catalyst system low efficiency what do i need to do to repaire this problem all help welcome

ST170 by any chance?

It's the pre cat lambda sensor that needs changing.

if its an rs running a decat then this can cause that fault.....just have to clear it every now and again

that codes saying it thinks the cat is fucked.

do you have a cat? with a cheater?
could also be the post cat sensor faulty.

is the code cat effinency below threshold if so more than likely cat you need to ckeck the lamba s if they are switching the same your cat up the swaney