Most people don't know how to diag evap system codes. It's really not that hard, just a matter of plugging off the right lines and using a smoke tester.
Problems that it could be:
1) gas cap
2) vacuum lines under the passenger side of the intake manifold
3) a broken/cracked nipple that those vacuum lines go to
4) Lines going to charcoal canister. It's going to be a SMALL leak. So look for damaged lines/hoses.
It's not going to cause you any damage, it's just annoying. I've had this code come up on my 99 Legacy. It was for the sensor that is on the passenger side of the intake manifold that the vacuum lines go to, the nipple broke. And yes I know that I just restated what everybody else said. I just figured organization to problem areas might be nice.