Engine/Transmission (1998.5 - 2002) check gauges

[Moe]

Going down the road and chime comes on and check gauge lite come on. Did the key check and it set a p1682. Got home and shut it off. Restarted and it started charging again. Dont know where to start. checked both batts and they are similar in charge, 12. 82 and 12. 87 volts. They only diff I can see is that when starting in the morning the battery charge pulls down further than it use to. Any ideas? thanks, Moe