Personally, I would tackle the p1633 first. The computer thinks that your voltage is extremely low and it is potentially a cheap fix. The computer controls almost everything, if it is not working properly then many other things may not.
If you can... clear codes and check them again. If the p1633 comes back, locate the ground point for the computer (it is right next to it in the engine compartment). Pull it off the firewall and clean it up thoroughly, sand the paint off of that ground spot and reattach it. Smear some dielectric grease over it to protect the sanded area. Hopefully that will solve that code. While you are at it you may as well tend to the other three ground points in there... one is right in front of the battery, another behind the battery on the fender, another on the firewall to the left of the brake boost. I think there are at least two more but I have not found them yet (haven't really looked to hard though).
If the OD button is working properly how did you determine that it is not going into OD? Not doubting you, just curious.
