clamchowder

Hi all,
I have a 2007 Accord LX, manual, w/184k. I replaced the catalyst converter no more than 3 years ago. Recently, I get a check engine light, it's P4020 according to a local mechanic. The guy told me catalyst converter usually lasts about 50k miles. I find that a little misleading. Anyway, the check engine light comes on/off intermittently (it comes on then off for a while then on again). Question, is my cat going bad already or is it the O2 sensors going bad? Both O2 sensors are original, haven't been changed. I use Mobile 1 full synthetic oil and my car is up for smog check.

Thanks in advance.