Lately, whenever I go for a long drive, my check engine light has been
coming on.  It goes off again the next day, before I have a chance to check
it out.

However, today I was able to have it read at Auto Zone, and it said P0170
(and the corresponding code for the 2nd bank).  Both banks are low, whatever
that means.  The guy at Auto Zone suggested either replacing the fuel filter
or the mass air flow sensor.  The light went out again before I reached
home.

Since the light is going out, should I be concerned?  Does his advice sound
warranted?  Thanks for any advice.