The P0170/173 are the codes that usually indicate a faulty MAF sensor, the P0455 is the code that prompted the message about the fuel filler cap. Obviously, check the gas cap first. Some other things that can cause the P0455 code are, in no particular order, the purge valve, fuel tank pressure sensor, and the charcoal canister shut-off valve. It's an easy system to troubleshoot if you have a hand vacuum pump and an SDS, OR some other means of activating the shut-off valve.