My check engine light is constantly on but will go out for a couple days and then come back on for 3 weeks and repeat the process over and over. I checked the code with a code reader and it said "warm up catalyst below threshhold". Does anyone know what this means and how to fix it?

 

O2 sensor I believe. I had that same code and when I replaced one of my O2's it went away.

 

I have the same thing, P0421 and P0431, on a 2000 A6 Avant. Are the catalytic converters warranteed separately somehow for a longer period than the car? Or is there another repair thatcan be tried before replacing the converters?