You would, most likely, get a CEL for a P0420 or 430 code. These could be your downstream 02 sensor, but if they are found to be ok, then the above codes mean that the precats are not working properly.
Hope that helps you out some.
Phillip Jackson
`98 Mystique LS
262K+ and counting...
ATX rebuilt @ 151K
"This storm has broken me, my only friend!"RIP Dime