Hi Sean

Almost without exception, every example of "P0420 Catalyst efficiency bellow threshold" that I have seen, has been accurate, in as much that the PCM has assessed the catalyst performance accurately.

Of course you have to look for any underlying issues, that might cause a new catalyst to fail, but it has to be said that you can place zero confidence in any aftermarket product. The introduction of "type approved catalysts" has done little to improve the situation.

Take a look here for some testing tips.....

HTH