My 99 Nissan Altima with 100K miles recently gave an emission control
system error code.  I remembered being in a rush when I filled it up
and thought it was just a gas cap that wasn't tight, so had the code
cleared.  Next time I filled the tank up, I drove only 5 miles and the
Service Engine light came back.  I assume it is the same code.

Any ideas what it might be?

Ray