P0421 after fuel pump
 

I have a '99 626 with the 2.0 liter and ~170K miles and just got the P0421 code. The car had been sitting for awhile so I had the fuel pump replaced and it was working fine for a week or two then I got the code. Would it make sense to start with plugs or could this only be the catalytic? Also, would i replace the front (expensive) catalytic, or the rear one? Thanks.
Tim