RE: error 631
You need to check the O/D button and harness. It sounds like it is shorted. It is a momentary contact switch to ground. If the O/D button does not work due to a harness break, I do not think you will get a code.
Do a search of the forum and I belive you will find instructions. Basically, you'll need to pull the EATC trim, remove some screws, remove the shifter trim and console to access the harness. I had to replace my button switch which does not require removal of the shifter knob, but if the harness needs replaced, you need to carefully remove the shift knob also.