The PO455 may or may not be related to filling up with the engine running. The self-test for PO455 is performed (initiated by the ME-SFI control unit OR Diagnostic Module) once per trip. It takes only seconds for this self-test to be performed. If you happen to be filling the tank while this test is taking place you'll set the fault code. If the PO455 code is set, the control unit will not perform the self-test for PO442 (small leak). If the vehicle "passes" the PO455 test, it immediately begins the PO442 test. Once these tests are performed and passed, you can drive without the fuel cap and not set these codes. The next round of testing begins with the next driving cycle. There are certain criteria that must be met in order for the self-tests to be performed. On earlier DM (diagnostic module) equipped vehicles the fuel level had to be between 1/4 -3/4. The vehicle speed had to be 0mph, and the engine had to be running for at least sixteen minutes (approx). As far as the duty cycle is concerned it's about 8% during EVAP system self-testing, and varies during normal tank vapor purging, depending on operating conditions. I could go on forever about EVAP system testing. I've seen SEVERAL different scenarios (including faulty/loose fuel caps) that cause the PO442 and PO455 codes.