My 89 740 lost reverse gear. (203K miles) It just happened without warning. Do I need to replace the tranny? If so are there any tips or procedures on how to do this? Where is a good place to find a used tranny? Thanks I appreciate.

It is a 1999 S80. I pulled the codes and 3 came up. P0172 which is Bank 1 system too rich and P0175 Bank 2 too Rich. Then P1633 which my scanner said Auxiliary something out of range.... That isn't exactly what it said but close. Does anyone know what this means and how to replace this? Thanks.