20 is the Electrical Load Detector or ELD. It's built into the fuse box.

41 and 65 are the primary and secondary O2 heaters.

Before replacing the alternator or battery, I would check the alternator/VSS fuse under the dash. I went through 3 batteries and 2 alternators when I had a similar problem. My alternator wouldn't charge my battery and eventually the battery would die. My speedometer also worked spradically. After swapping batteries and alternators, I checked all the fuses, and the VSS/ALT fuse under the dash wash out. Replaced that, and everything was gravy. If there's noting wrong with your O2's and your VSS/ALT fuse is out, replace it and reset your ECM. Your problems should go away.