2008 Pontiac Grand Prix, a P0108 code, weird voltage to MAP sensor

2008 Pontiac Grand Prix, Base model, 3.8L V6 engine.

I'm dealing with a P0108 code, which indicates that my MAP (Manifold
Absolute Pressure) sensor isn't working properly. I took a multimeter (DC
voltage setting) and tested the 5V line with key on, engine off. Instead of
reading at 0V to 5V, my multimeter reads OF (or 0f), which I think means
overflow or overload. The multimeter (generic Radio Shack model) isn't
broken, as it can read correct voltage values from things like AA batteries
(tested and working). I'm scared that there may be another problem in the
electrical system that's causing my MAP sensor issues.

Anyone have suggestions on what I can check next to ensure proper voltage
to the MAP sensor?