Have you unhooked the battery cables any time recently?
Are they loose if you wiggle them.
I'd suggest pulling both, removing the bolts out of the rubber and pulling the boots off for a detailed inspection.
My first acquaintence with this lovely code was a few months after swapping a battery in the car. A few months later P0171 was a nemisis. Turned out my battery cables were corroded badly, pulling the boots made it obvious that the corrosion went down into the wire below the sheath of the wires. My fix ended up being replacing the wires for all battery cables.
00 Regal GS: All kinds of cool stuff.