This last week here in SW Arkansas we have been below freezing. I didn't start the car for 2 days and when I tried I had zero battery. Battery was new in August so I took it in to O'Reilly's and they gave me a new battery. After I hooked it up it started right up and I just drove it a few blocks and everything seems ok. But the CEL is on so I hooked up my OBDII reader and got these codes:
B1342 - Body. which come back as defective ECU. But if ECU was defective would it run and drive good?
P0563 - Powertrain which is system voltage high possibly alternator. Alternator was new in August same as battery. I checked the voltage at the battery. Battery show 12.3 volts, Ok as a battery right out of the store isn't usually really fully charged. Started car and voltage at battery ran from 14.1 to 14.26 volts.
Doesn't seem like the cold temps should have caused these problems as these cars run in the northern winters without problems.
Now I just went out to try it again and I must now have a bad fuse as none of the dash icons come on with key on/engine off and neither does the display that shows odometer etc.
Any ideas anyone?

 

I'd guess, guess the first of your listed codes was right ... bad brain.

 

Sometimes a poor connection will function borderline until the temp drops. Then it now appears and needs to be corrected.
I would suggest checking and cleaning the battery cable connections to the vehicle especially the negative (Ground) cable.

 

I was told by a friend to disconnect the negative battery terminal for at least 20 minutes . That would let any residual power in the ECM bleed off. Then when I hooked the cable back up the ECM would reset and do it's normal diagnostic routine. So I did what he said and when I turned the key back on everything was back to normal. All icons showed and when I started it up the check engine light was gone.
Seems like the whole reset thing would have happened when I had the battery out of the car when I went to get it replaced. Who knows. I'm happy and thanks to uhoh and hanky for their replies.

 

Thank You for the update.