NS swb v6 trouble code PO421

[djr75]

Hi out there,

Im the proud owner of a ns-x swb v6 that i have babied, always serviced and runs on 98ron fuel since new. It still, looks, smells and drives as it did when it left the dealer 72k km ago though now has the check engine light is up and showing the trouble code PO421 (warm up catalyst efficiency below threshold bank #1).
Has anybody out there seen this issue in the past? I find it hard to believe that one of my cats have died???

[Hobbsy]

G'day. How did you go with this one, my NS is throwing it every couple of days. Thought I had a manifold gasket leaking, pulled manifold found cracks. Welded up, refitted, spat code today.