if your vacuum lines are ok, it sounds like your purge control solenoid might be seized. i had one last week that was seized, but was ohming ok.
start with getting two wires and disconnect the valve. hook up the valve to the battery and if you hear a soft click, keep looking. if you don't hear(it's soft, you'll most likely feel it better) replace it.
(usually the valve just seizes. replace it and you're done. if it clicks and you're not sure about it, try to vacuum test it. hook a vacuum gauge up and see if it holds about 15" of vacuum. if it does, energize the solenoid-the vacuum should drop to 0 immediately. don't exceed 15" though, or you could damage it.)
now, if the purge control solenoid is clicking(and testing ok if your absolutely thorough) check the purge flow switch. this thing is just a pressure switch.
if there is positive pressure it is ON(should have continuity) it sends a signal to the ECM to energize the purge control solenoid. the ECM will then energize if other pre-determined conditions are met which we have no control over.
if there is negative pressure it is OFF(no continuity) the ECM detects no signal from the pressure switch.
this means 2 things. either the pressure in the evap system dropped because of recent purging and normal operation dictates it's time to shut of the purge control solenoid or there is a malfunction.
the malfunction is the ECM determined it's time to energize the purge control solenoid and the purge flow switch is not changing after 6 seconds of operation. it should change value to the ECM but if it doesn't it will throw a code cuz it doesn't know whats happening.
oh well. hehe...
SO, check the purge control solenoid then if it's ok, check the purge flow switch. let me know if you need some help or something.
This code is for the canister side of the evap system. the gas cap fixes a tank side leak usually. (P1456)