12.3 volts means the battery is close to fully discharged, but it should still run the head lights. First charge the battery at a slow rate for an hour or two, then try to crank the engine. Assuming everything is still dead, follow the smaller battery positive wire to the under-hood fuse box, and be sure that connection is clean and tight. Follow the smaller battery negative wire to the body and be sure that one is not rusty.
Also don't overlook loose or corroded battery cable connections. Turn the head lights on so current is trying to flow. Measure the battery voltage right on its posts, then move the meter's probes to the cable clamps. You should get exactly the same voltage at both places. If a cable clamp is loose or corroded, the second voltage will be lower.
Wednesday, May 10th, 2017 AT 10:31 PM