2004 350XG Check Engine Light - Evap. problem

I have had my car into the dealer twice in four months for the check engine light coming on. First time they said it was an evap problem, they repaired
and cleared light.
Today, had it back for the same check engine light and was told it was an evap. problem code came up as DTC PO456....however, this time they said they could find nothing wrong. So, they cleared the light...
What is my next step??? Dealer said they find nothing wrong and if it happens again they will have to call Hyundai for help.............Is there just something they are overlooking?
Thanks for any information you can give.
P.S. Car has 28K on it and other than this has been great for me, no other problems.
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P0456 means there was a very small evaporative leak detected.
The Hyundai scan tool provides the technician a means to have the vehicle run the evaporative emissions test. If the test repeatedly reports back that there are no leaks found, then that means the problem isn't present at the time the technician is testing the vehicle. In that case you're probably looking at one of two possibilities. First, the fuel cap could have been a little loose or crossthreaded, or the tether could have been caught under the cap gasket. I've seen all of these. Second, the canister close valve may intermittently not seal properly. This valve doesn't affect the function of the evaporative system except that the vehicle cannot properly test the system. The sole purpose of the close valve is to seal the system so that the computer can apply engine vacuum and monitor the fuel tank pressure sensor to check whether the system is leaking. I've also seen a few of these close valves fail in this way.
In most cases, if I run the evap test 10 times and receive no failures, the car doesn't return with a repeat issue. At this point, I'd recommend you make sure that the tether doesn't get caught under the fuel cap, that you don't crossthread the cap in the filler neck, and that you fully tighten the cap. Then, if you're diligent about this, and the same code returns, you'll know that the fuel cap installation isn't the issue. In this case, be sure to represent to the dealer how diligent you were in making sure the cap was installed correctly.
The close valve is the only moving component in the system which could cause a small leak, so it's the primary suspect in causing an intermittent leak. As I recall, though, I did see a few Santa Fes where the screws for the fuel pump were overtightened and distorted the flange so that it didn't seal properly. The leak was apparently just the right size so that the code would only set on 30% of the evap tests or so. But, once again, running a few successive tests caused a failure to occur.
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