If your NOx emissions are high then you are right it is probably the CAT. How bad is it? I just finally passed my SMOG test yesterday and was having the same problem. Try retarding the timing to 13-BTDC from the stock 15. That is how I passed. At 15-BTDC my NOx was 948ppm and at 13 it went down to 787. Max here in CA is 799 this time so I barely made it. So if you are within about 150ppm retarding the timing should do the trick. It lowers the peak cylinder temperature which reduces NOx emissions. Hope this helps...