First, this is an emission problem and the dealer will cover this for up to 8 years or 80K miles.
You can have OnStar clear the code and see if it returns. This will get you out of low power mode. Or, your local auto zone can clear the code for you as well with a basic code reader. Clearing the code will get you out of reduced power mode. DO NOT UNPLUG THE BATTERY to clear the code. You will create more problems than good.
Take this into consideration... If you clear the code, you make it alot more difficult for the technician to diagnose and repair the problem. Also, if the problem is a hard/current fault, the code will come right back and you will have to get it fixed before normal performance is restored.
Code P0449 is a fault detected with the vent solenoid on the EVAP system. An EVAP system is a vapor managment system for your fuel fumes. When you fill the fuel tank, the fuel vapors are stored in a charcoal canister. When the engine is started, the vapors are purged to the engine for combustion using engine vaccum.
The Vent solenoid is a solenoid that opens the fuel tank vapors to allow the tank to be filled or purge fumes to the engine. The solenoid closes so the engine computer can test the system for leaks. Much more complex of a system than you would likely like to hear about.
Basically, your problem is most likely a vent solenoid that has shorted due to water intrusion. In fact, GM has released updated parts for this repair so the problem does not reoccur.
Do you have to go to the dealer? No. You can test and replace the part yourself.
1. Check the TCCM-battery fuse 14 in the under hood fuse block.
2. Check for Battery Voltage (B+) at the vent solenoid pin B Red/White wire.
3. Check the White wire at the vent solenoid for a ground with the vent solenoid commanded off.
4. If the White wire has a ground with the vent solenoid commanded off, unplug the C1 connector at the Engine Control Module (ECM). If the ground is gone replace the ECM, if there is still a ground on the White wire with the ECM unplugged, check for a short to ground between the vent solenoid and the ECM.
Battery voltage is supplied to the Evaporative Emission Control System (EVAP) canister vent solenoid valve. The control module grounds the EVAP canister vent solenoid valve control circuit to close the valve by means of an internal switch called a driver. The scan tool displays the commanded state of the EVAP canister vent solenoid valve as ON or OFF. The control module monitors the status of the driver.
The 5V reference signal is not involved in this fault. I do not know why they would tell you that.
But, since this is an emission problem... it falls under the emission warranty.
The dealer will take care of this free of charge for 8 years or 80K miles, whichever comes first.