P0420

I have a P0420 [CAT system is operating below threshold] show up whenever I drive on the motorway, and it always seems to be M6 south!! daft I know, doesnt do it coming north, or on the M56!!

So, around january, car had a new cat pipe, it was good type approved one, I am not saying that its not at afault, just stating that it was new

I have a code reader, what can I check for to see if its the post cat sensor, the cat, or the pre cat sensor?

ta

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P0420