what causes code p449??? on 2004 Chevrolet Silverado 1500

I have an 04 Silverado 5.3 extended cab. Not flex fuel. My CEL went on showing code P449. I have replaced both the vapor canister and the solenoid valve. I checked all the hoses for obstructions. The scan tool i used said something about the evap circuit low. Could this code be due to a bad gas cap???? I dont know what else to try????

This code not consistant with fuel cap or leak. I assume when you say solenoid valve that you mean the purge valve solenoid. Problem seems to be with that or circuit. Check your fuses and see if one specifically for that valve. You can also use a volt tester and check and make sure you have power to that circuit while truck running.
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Had the same issue, I cleared the code and took the gas cap off and put it back on. Made sure it was tight. Problem resolved......