2004 Chevy Silverado tighten gas cap

P0455 is a large leak detected if gas cap sealing surface is ok most likely cause is the evap vent valve it is a valve usually round located in front of the fuel tank or at rear. Electrical connection at valve rarely fails and all you can really do is inspect it visually its a red connector hooked right to the valve. With a tech 2 you can command the valve on and off and hear it click im fairly sure you dont have access to one so all you can do really is check the power supply to the valve you wont be able to check the control side that would be the ground. You could remove the valve and put power and ground to it and listen for a click but 99% of the time the vent valve is the cause at least in my experience. Gm replacement valves are of a new design you have to cut the plastic tube off the vent valve that goes to the evap cannister and hook it to the new valve with heater hose and clamps gm also wants you to run anther piece of hose off the vent valve to the top of the transmission with the provided filter clamped to the end.I dont believe this is completely necesary I usually just run them up the frame and zip tie them to the fuel lines right behind the abs module pump.