PO450 is not listed in the service manual but the generic OBD code reference states 'Evaporative emission control sensor malfunction'. Looking at the evaporative emissions system in the manual it would appear that the combined fuel level/pressure sensor is the only such sensor shown. Since 450 specifically states a 'sensor' malfunction I would check the connections on top of the tank. There are detailed diagnostic tests in the manual for the sensor which may be worth following, but I would hope that the dealer has done this already. If you don't have the manual I would suggest you download it and have a read of the emission systems section before conversing with the dealer. I would also check that the filler cap is sealing correctly (there's a valve in there too which opens at specific vacuum and pressure levels too) as that has been known to throw codes, albeit not PO450.
HTH