2001 Honda Civic Check engine light

I drive a 2001 manual Honda Civic with 170k miles. Recently, the check engine light came on. It has done this in the past, but has always reset itself after 3-4 drives. However, this time it's been on all week. I had an auto store run diagnostics on it, and the result was a code the man had never seen. It said, "vacuum in tank cannot pull." He had no idea what that referred to, but thought maybe it had something to do with the fuel filter. Today the light reset itself after I stalled out at a light and had to restart the car. Should I dismiss it now that the light has turned off, or is it something that I should be concerned about still?