1993 Cutlass Ciera won't pass emissions testing

I have a question. I live in the state of Georgia and they require an emission test every year. Last year, when I moved here, my 2003 Cutlass Ciera did not pass emission--it needed a catalytic convertor. It was tested again and passed. This year, I got it tested and it failed---I put on another catalytic convertor and it still failed. they do a 25/25 Test and it passed on the NOx ppm..my car's reading is 0313 and the allowable is 878, but on the 50/15 test it failed. My car's reading is 2596 and allowed is 970. Someone told me to check the rpg valve, but it doesn't have one. Any ideas what this could be?