Vent solenoid location

Purge is on top right of engine near fuel injector connector.
Vent is on crossmember rear of fuel tank

Did you check the block box connected to the vent solenoid? There's a hose that comes off it that appears to run nowhere, that's the fresh air vent. If it's blocked it won't allow air into the system which will set the trouble code. If that's clear, the vent solenoid itself is bad and needs to be replaced.

I would just replace the valve if the connector is ok and not corroded.

P0449 BUICK Description The Engine Control Module (ECM) monitors the Evaporative Emission System (EVAP) Vent Valve/Solenoid system wiring for opens or shorts. Could be a problem with the vent solenoid or other component of the fuel tank venting system.

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P0449 control vent valve/solenoid circuit malfunction, This code necessarilly deal with replacing the vent solenoid it about the circuit,the evap vent solenoid circuit may have rub through on the frame (located on the back of the fuel tank),it can also be the connector,the vent solenoid itself,or the connector

There is a solenoid that open and closes with 12 volts, usually located close to the fuel filler tube area. This open to allow you to fill your tank, then closes after some inputs for the computer are satisfied, and then a pressure sensor verifies the system is working. First, make sure the cgas cap is tight, and if the light doesn't go away in a couple days of normal driving, get the solenoid checked.

Code po449 is for bad  solenoid vent valve.After replacing the valve  check for the   12 volts at the "pink" wire.
This error relates too 

The PCM has determined that a malfunction exists in the electrical circuit for the vent valve/solenoid in the evaporative emission control system.

Definition:
EVAP (Evaporative Emission System) purge/vent solenoid circuit condition

Explanation:
The solenoids and system wiring are monitored for opens or shorts

Probable Causes
1- Purge or vent solenoids defective.
2- Check connector and wiring.
3- Fuel saturated vapor canister.
4- Failed EVAP vent solenoid.

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The cost of the vent solenoid valve is near to 30 bucks. And the valve is located near the gas tank.

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Hi

This engine code is related to a malfunction in the electrical circuit for the vent valve/solenoid in the evaporative emission control system.

Probable causes are;

1- Purge or vent solenoids defective.
2- Defective wiring or bad connections in the vent valve/solenoid circuit.
3- Fuel saturated vapor canister.
4- Failed EVAP vent solenoid.

These engine codes are useful indicators to where the fault lies but usually require further troubleshooting to fully determine the underlying cause.

However I have listed the probable causes so to give you a starting point for your investigations.

Hope this helps. Please remember to rate my answer. Thanks.

Check all vacuum lines from the gas tank to the evaporative charcoal cannister by the gas tank and to the purge valve located at the engine. Check the electrical connector at the vent solenoid which should be located on the charcoal cannister and purge valve solenoid by the intake manifold.

P1646 - Evaporative Emission (EVAP) Vent Solenoid Control Circuit