If you are not getting a 5-4-1 code along with your 3-1-5 code then you can assume the evap valve is getting its electrical signal from the ECU. Check that there is good vacuum from the engine to the evap valve by removing the inlet hose fromt he evap valve and feeling for a vacuum in the hose. After a minute of idling the evap valve should open to purge the evap canister, so after a minute you should feel a good vacuum from the hose going from the evap valve down to the evap canister. If you don't feel a vacuum, and you know the hose is not blocked, then you can assume the evap valve itself is bad. Thats about the only testing there is for it. Good luck ..... Mike