Check engine and slip indicator lights turned on with Code P0420

I have the truck listed above SR5e. The slip indicator light and the check engine light both came on. I did a scan and the code was Bank 1 and P0420 which is Catalyst system is below threshold. I replaced the oxygen sensors and cleared the code. After about 30 miles, the lights came back on. My non-mechanic friend suggested that my engine might be using oil and was fouling the sensors. I thought it might be a logical try. Although there was no smoke and no drop in oil level, I tried some oxygen sensor spacers to get the tips out of direct flow. Didn't work either. The lights still came on. I did another scan and the new sensors seemed to be working and the upstream sensor tested out as 770.0 F and the down stream sensor was 532.3 F on warm up.
Once cleared, both lights come on and remain on after about 30 miles. Any suggestions?