Vent valve circuit faults
MIL on with P0449 stored, there will likely be other evap codes stored, such as P0442 or P0455.
An electrical fault in the vent valve circuit is the only way for this code to set. The other stored evap codes are likely symptoms of the P0449.
Use a scan tool with bi-directional capabilities to command the vent valve on. If there is an audible 'click' from the valve, the fault is not present at this time. Try commanding the valve on and off several times and leave it in the on state for a few minutes to attempt to recreate the failure. If there is no operation from the valve, disconnect the electrical connector at the valve, back probe both wires and connect each lead of a DMM to the back probe pins. When the valve is commanded off, the meter should not read any voltage, when the valve is commanded on it should show battery voltage. The most common fault is a shorted valve but wiring problems are fairly common as well.
If the meter does not show battery voltage at any time the most likely problem is with the power supply side of the circuit. Using a wiring diagram, determine which wire supplies power to the valve. This wire should have power with the key on. If it does not, check the fuse, then check the wiring harness leading back to the valve.
If the meter shows battery voltage when the valve is NOT commanded on (as well as when it is on), then there is a short to ground on the control side of the circuit. Change the DMM to ohms and measure resistance from the control side of the connector to battery ground. When the valve is off, there should be no continuity to ground. Most control circuit failures will have resistance of several hundred to several thousand ohms. First, remove the key and unplug the PCM connectors. If the circuit goes open, the fault is within the PCM. If there is still continuity, follow the harness toward the rear of the vehicle, checking for shorts to ground and inspect each connector along the way for corrosion, bent pins, etc.
If there is no control signal (ground) at the vent valve connector, verify control signal at the PCM. If the PCM has the ability to ground the wire, check for broken wires or bad connections along the harness. If the PCM cannot ground the wire, it is bad.
The most likely locations for wiring problems (power or ground) will either be in connectors or, on some 4WD models, the harness can short on the front drive shaft.
Repair any wiring or connector faults. Replace the vent valve and/or PCM as necessary.
Note: if the fault was constant ground on the control side of the circuit the vent valve should also be replaced to prevent future problems.