Code PO449?

Hello, this code is for the Evap system Vent valve solenoid circuit malfunction. The vent valve is in back near the gas tank, it closes off the Evap charcoal canister when the system runs a self-test on the Evap vapor management system, which pulls fuel vapor from the gas tank so it's not released into the atmosphere into the intake manifold and it's burned with the air/fuel mixture. What was the original reason AutoZone said you needed a Purge valve and knock sensor? Were there codes for those 2 components? That is the only reason I can see them trying to sell you parts. It might even have been an Evap leak detected code.
Do you remember them scanning the vehicle for codes before they sold you the parts?

This code can set if the Vent valve in back is not closing, meaning a failed vent valve, or it can be a wiring issue. Due to the location of the vent valve, they are exposed to the elements and dirt and salt from the roads. You can try just replacing it and hope it's not a wiring issue. The reason the code came back right after you put some gas in is because there needs to be 15 to 80% amount of fuel in the tank for the Evap to run its self-test. I'll try to find a diagram for it, but it's attached to the black canister near the gas tank in the rear.

The first diagram is the entire rear frame of the vehicle, the Charcoal canister is circled in yellow. The 2nd diagram shows the location of the Vent valve (in green). Diagrams 3, 4, 5 are for replacing the Vent Solenoid. I'm not sure why they want you to cut the vent hose, I would get the Vent valve first then take a look at what's needed to replace it. Make sure to check the electrical connector for any dirt or corrosion, also check the wiring to the connector to ensure that it's not rubbing against the frame and shorting out or cutting into the wire harness.
A capable scan tool is required to bidirectionally close the Vent valve to check the system for leaks and proper sealing.