Chevrolet Repair: vent valve, vent valve, gas mileage

Question
I have a 2002 Cavalier.  My check engine light has come on twice, got code 0440: evaporative system fault.  I got it tested, their smoke test showed that my gas doesn't fit tight, so they replaced it today.  They also found that a small amount of smoke came out from the "vent valve", and that if the light comes on again this will likely be the reason.  Is this a problem that definitely must get fixed, will it affect my gas mileage, and is it something I could replace myself with minimal car fixing experience?

Thank you!

Answer
Hi Lindsay,

Is this hard to fix?  It is not in the easiest place if on the tank, or if under the hood, then it is fairly easy.  

Special tools?  Standard tools should do it.  It is pressed into a ring seal.  Pull/pry hard and it should come out.

Is it necessary?  Where do you live?  Some areas require a yearly inspection of vehicles.  You won't pass if you bypass this system.  If the test is not required, then you can eliminate it.  The reason for this device is polution control.  However, it makes so little difference, it is a farce.  

The light is on when it senses a lack of pressure in the tank.  

I would fix the problem rather than bypass it.  

C J S