Old thread but I just had this problem, and looked through seemingly every thread on this site and didn't see the issue I was having mentioned, so I thought I'd put my finding here.
For me the passenger side wouldn't lock using the fob or the switch on the either door. I could hear the solenoid, normal when unlocking, but I'd hear 4 clicks of it on the passenger side when I hit the lock buttons. I was getting a B122D29 (Lock unit for central locking Implausible signal) error in obd11.
My issue had started when I took the door panel off to replace the outside mirror as some evil person had broken my mirror presumably trying to steal my mirror cap.
After much investigation my issue was pretty dumb, sort of unlikely, and sort of surprising to me. It turned out that while I was trying to figure out how to disconnect the cable from the latch mechanism to the interior door handle, I'd pulled the sheath of the cable out of the housing of the latch mechanism. It sounds barbaric, but I really was trying to be careful and I didn't pull it that hard. With the cable sheath not seating, it effectively made the sheath too long for the cable to fully retract even though it seemed to work fine to open the door. This was, I believe, not allowing the microswitch inside the latch to release — the mechanism won't try to lock when the handle is being pulled. I just had to push the cable sheath into the housing/recess in the latch and everything worked.
