I'm by no means an expert on ECU's/sensors, but my '98 Lumina did this a few years ago. In my case, the battery was still able to crank and start the engine (barely), but gave a low reading on the multimeter (barely 12 volts [should be 12.4 - 12.65]). This was enough to activate the cryptic 'Volts' light.
I'm not sure you have the same problem. . . I just recommend having the battery tested (if you haven't already) before spending a small fortune on the expensive parts you mentioned. Auto Zone, O'Reileys, NAPA, and other stores that don't actually work on cars are the best. They only run the tests you ask for, and never try to sell you something you don't need.