98 Chev Blazer P0420 Code

I have been getting this code P0420 (Catalyst system out of spec) on a regular basis and discovered with a CarChip E/X (Provantage.com has the best price I have seen) everytime the code comes on my Fuel System Status goes from a closed state to an open state while the car is idling. Never happens unless I'm idling and after I have driven for a while. I have replaced the two pre O2 sensors. I have the post sensor on order (best price AutoPartsGiant.com) but not installed yet. Plugs look good. Car runs fine. No rotten egg smell. Anyone have any ideas why the fuel system status is going from closed to open and then triggering the Service Engine Soon lamp? Car has V6 4.3l W engine and 80k miles. Thanks for your help...much appreciated. Great forum!!!

A code P0420 is checking how good your convertor is working. I would call the dealer and ask if they have a PCM update addressing a false P0420. If your S-10 isn't included in the reprogram then you will most likely need a convertor or HO2 sensor.