2007 Acura MDX check engine light

Just wanted to share, in case others have the same issue with the 2007 Acura MDX SUV. At 62K miles, the Check Engine light came on with a note that said Check Emission System - Error P0420. I read several websites about this and it appeared to be related to O2 (oxygen) sensors, the catalytic converter, or simply not tightening the gas cap enough. I noticed that the previous 5 fuel fillups had been at either Safeway or Costco. Actually, the check engine light came on about 30 minutes after filling up at Costco on a road trip up to Vancouver, BC. So my first two thoughts were, maybe I didn't tighten the gas cap enough and maybe my vehicle dislikes Costco gas. But as I drove longer, the light eventually turned off (during the same road trip). Maybe the car warmed up, who knows. Unfortunately, after letting the car sit for a few days, the light came on again the next time I drove it. And it remained on for the next 10 days or so. The next chance I had, I fueled up at Chevron and tightened the gas cap many clicks past what I normally do. The light seemed to go off almost right away. So either my car loves Chevron or it really was the gas cap. It has remained off for the past 2 weeks, even after letting the car sit a few days. My goal was to avoid taking it into the dealership and to wait it out. The last time a check engine light came on (for the 4WD system), I was told that all they did was clear the codes and that it looked like nothing was wrong. I was still charged a $120 diagnostic fee. So anyway, hope this helps someone out there. 62K miles seems really soon to be replacing the catalytic converter, so I was hoping it was a much simpler, cheaper solution.