CEL 31 is my freakin CO-PILOT.
I have had that bastard on my dash for over 2 years now with no ill effect. I have changed my Purge solenoid and vent solenoid and have had some wack ass machine blow SMOKE through the EVAP system to try and find this leak. In my situation, it is more than likely the roll over valve on top of the gas tank where you cant get to it. When we blew smoke up my cars ass, it was coming out from the top of the gas tank, but I've since checked the fuel pump/gauge sending unit ports and they dont leak.
Here's what I CAN tell you about this bitch:
*If you reset your CEL, it may take a few days to come back on, it may take a minute, so don't get too excited the first time it vanishes.
*If you want to try to replace your solenoids, go to a salvage yard and pull them. Might cost you $10. (Compared to MItsu who wants $197). In fact, I have the original one that was not the problem in my closet right now. You want it? Let me know.
*Break down and check your hoses. (I still haven't done it thoroughly enough, but...)
BEST OF LUCK! I hope you can get this fixed mang!