97 Elantra and Code P0455

I have a 97 Hyundai Elantra with a 1.8 DOHC motor with 150K miles on it. I am getting ODB2 error code P0455 and the check light is on. I am using a carchip from davisnet.com to read the codes. I have reset
the light once but it came back on in like three days or so, about 100 miles or so. I had the car in for the gas cap recall and they replaced the cap. I think that was like 30 or 40 K miles ago? The car appears to run about the same as it ever has.
Should I just get another cap? What else would cause this code.
Thanks for your time/help with this.
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This code means that the car cannot vacuum the evap system during its self test. This can be caused one of two ways, either by a leak (such as a missing/loose/defective fuel cap) or by preventing the vacuum from reaching the tank and sensor (possibly due to stuck closed purge valve or other blockage).
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Used the shop manual and checked the purge valve. The valve resistance reads good and moves when voltage is applied. Don't know if voltage is getting to valve?
When should I see voltage to valve? Still need to check overall vac off motor. Also need to see how much new flue cap from dealer is.
Thanks for the help so far.
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