Getting P0455 DTC code

[MatPorsche]

I have a 2004 Porsche Boxter. Recently the Check Engine Light has started coming on, usually soon after I have refuelled. I have a generic code reader, and it shows P0455. I can clear it, and it usually doesn't come back on until I refuel again.
Any ideas on what is wrong?

[JFP in PA]

Bad gas cap or disconnected hose on EVAP unit. Usually the gas cap, which is also the cheapest and easiest to replace............

[MatPorsche]

Would that not show a P0457?

[JFP in PA]

From Porsche's OBD II manual"

"P0455: Fuel tank pressure below lower limit

Possbile cause: Loose, missing or defective gas cap
Vent lines disconnected from EVAP canister......."

I could go on, but I think you get the picture........................