p0420

Hello, I have a 98 Silverado with a 5.7(350), my service engine light keeps coming on and always reads the same code, p0420 which means"catalyst system efficiency below threshold (bank one)", I have one of those hand held diagnostic scanners. I have changed both oxygen sensors on the drivers side( I have a total of 4 on my truck) and I have used G.M. parts. I usually get a day of driving before the light comes back on, then I clear it and about a day later it is back on. The truck runs the same as it has always, no difference in fuel economy and my catalytic converter is not plugged or is any thing rattling in there, idles fine ect. Any info would sure be appreciated, thank you........Ed