keyless entry touchpad problem

mmcenany's picture
1993 - 1998 Nissan Quest

My 93 has recently begun to lock all the doors whenever it wants. Usually it's whenever the passenger door closes but not always. As a precaution, I've tried to enter a new code into the keypad following the instructions in the manual. I have the original code, but I can't get it to work. Am I missing something? I bought it used so I don't know if the code was ever reset or not.