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Hi, I can help you with this. I have a service news article here and have actually seen this failure before. Open the fuel door and see if you can smell gas in that area.
Here is the info. If everything else is ok, this is going to be the problem
Troubleshooting DTC P1456
Troubleshooting DTC P1456 (EVAP system
leakage [fuel tank system]) in a '03-04 Pilot? Pinch
off all the hoses between the fuel filler neck and
the fuel tank, and retest. If the leak stops, replace
the fuel fill cap first. If you've still got the leak after
replacing the fuel fill cap, then replace the fuel
filler pipe. There's an O-ring between the plastic
and threaded part of the filler neck that may be