Thanks you two. The box I own to read the codes can reset them too,
so I erased them all and the 'check engine' went off. I just drove
for about 15 minutes and the engine light didn't come back on.
However, I have a gut feeling that this one will come back since it
was on for about a week...

P0420 Catalyst System Efficiency Below Threshold

By the way, what should I be doing about P0420 when it comes back as I
except it shall.