Can anybody help with a fault code that I keep getting about once a month and seems to happen under all types of driving.

The car is a Focus 1.6 Zetec and the year is a 2001.

After seeing the emission warning light come on I check the fault code and it always reads

PO420 and this reads to the book as catalyst efficiency below threshold BANK 1

As there seems to be about 20 warning codes that apply to the catalyst can any body tell me what bank 1 means I think it may be the first 4 cylinders as a V8 car would have a bank two ?

There are other warning codes in the book reference the catalyst showing a Bank 2 does

bank 1 and 2 refer to the first and second oxygen sensor before and after the catalyst.

I have checked the whole exhaust pipe and no sign of any leaks were found ?

I reset the warning light and erase the code and nothing happens again until about 100 miles are covered and then on comes the light again.

Hope someone can help.

Possibly a lamda sensor going down??