The 401 sounds like the DPFE sensor is bad - or a hose is broken going to it - need to check into this more.
The 455 is like 06silvercrew said, something with the gas tank is leaking air into the system - as in, it's open to the atmosphere. Need to check all the hoses, make sure the gas cap is on tight, AND that the filler neck isn't rusted through.
If you want to work more on it, and can do the work yourself, we'll work with you on it
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P0401 = take a vacuum pump, put it on the top of the EGR start the vehicle, pump the gauge, if the idle changes or the truck dies, 99% of the time = DPFE Sensor. Check the two hoses going to the DPFE as they tend to be brittle and break also. Havn't ever seen a EGR issue with those yet.
P0455 can be tought as it could be a loose gas cap (make sure it clicks) maybe a bad cap, have seen the EVAP unit (cant think of the proper name right now) the big black plastic deal by the spare tire that is by the fuel tank (I have seen those broken before) Check the fuel filler hoses. Being it's a large leak you should be able to find something visually, unless you have an EVAP solenoid stuck open.
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