fault code PO420 bank 1

[getfocusworking]

I have a 1.6 Zetec Focus 2001.

Fault code po420 has shown which is listed in the book as catalyst system efficiency below threshold bank 1.

Have reset code a couple of times but shows again.

Can anybody tell me is it best to renew the catalytic converter first or the first and second oxygen ( lamda ) sensors first.

Thanks for any help.

[SierraYankee99]

I'd suggest first getting technical advice from Ford. I'd suggest its just Lamda sensors rather than the cat itself.

[getfocusworking]

I'd suggest first getting technical advice from Ford. I'd suggest its just Lamda sensors rather than the cat itself.

Thanks for the advice.

Have taken the two sensors out today and found one had rust on the end of the sensor.

Have cleaned it off but if it lights up again will replace it.

Many thanks.