Really high CO emission following Cat/O2 sensor replacement

what were the numbers? EGR would not greatly affect CO levels. sounds like you have a malfunctioning (or unplugged) coolant temperature sensor(ECT). A faulty ECT makes the engine computer(ECM) think the engine is cold (does not warm up), this causes the ECM to run the engine in enrichment mode all the time, thus increasing CO levels (also poor fuel mileage and a dark stain on the rear bumper near the tail pipe). also, EGR operation occurs when the engine is fully warmed up. with a malfuntioning ECT, the ECM doen't see the engine warm up, thus not commanding EGR operation and increased NOx levels.