Adding this post in case others run into similar problem. Have a 2013 LR4. Recently, my rear passenger side door would not lock despite using the remote to lock or the locking button on driver side door. I have the passive entry system (door automatically unlocks when you pull handle and key fob is w/n close proximity). I would lock the vehicle and there would be no indication that anything was amiss, except when you pulled on the rear passenger door handle, it would always open. The alarm would not go off either. All other doors locked fine. Could not find a thread directly on point. There are a couple of youtube vids that I viewed that eventually helped me piece together how to resolve the issue (see below).

I first looked at fault codes, then tested the different fuses for doors, then i tested the lock actuator itself until finally determining it was the passive entry sensor in the door handle. An OE replacement handle runs about $600! Instead, you can just disable passive entry system by either two ways: (1) HARD WAY - physically disconnect the wiring of the handle to the locking actuator, which will require you to take apart the door card, remove the window actuator and door speaker, but you won't have to remove the locking actuator, or (2) EASY WAY - disable the passive entry system by opening the driver side door, have the key fob near the steering wheel and press the headlight button 3 times and then press the unlock button (Had to try it a few times to get it to finally disable). To turn back on passive entry, just press headlight button 3 times and then press lock button on key fob. Both techniques worked and neither caused any issues with operating anything else, like the other doors, folding/unfolding side mirrors, etc.

Original fault code for the passive entry handle sensor failure was under ECU for "PDM-Passenger Door", specifically B110B-11(6C) rear passenger side central locking motor-general electircal failure-circuit short to ground. There were some additional electrical faults for side mirror motor.

If you absolutely need to test whether your lock actuator motor is the problem, you can remove the door actuator and pull out to expose the wiring for the actuator, place on a stool or high table next to the car, replug in all the wiring to the lock actuator, and plug in the wire for the window actuator on the door card. The wire plug for the window actuator is the only wiring on the door card that needs to be plugged in to test the lock actuator otherwise the car will think the door is open and won't allow you to do anything. Once you've connected everything, use the shaft of a screwdriver to press in the latch on the lock actuator inwards all the way to simulate a closed door. Then use your key fob to try to lock/unlock. If working properly, there is a black metal lever and a white plastic lever on the side of the lock actuator; it should move about 1/2 inch to about 45 degrees when unlocking properly. I tested a bad lock actuator with a bad motor and it only moved the lever less than 1/8 of an inch.

Here are youtube videos that are helpful:
How to take apart the rear door card: How to remove rear lock actuator (front is slightly different, they have a vid for that): Another video on how to remove rear lock actuator, helpful to see different people at different angles: How to test your central locking system:
There is a video that other threads mention, but it only really applies if the white plastic lever on the lock actuator broke off. Not helpful if your lock actuator motor dies (which apparently is more common) . Providing it here to save you time from chasing down the wrong rabbit hole:
Part numbers:
Lock release cable, left/right: LR013916
Door handle with passive entry, left/right: LR030195
Rear door lock actuator, rear right, single lock, w/o dead lock, keyless start, passive, central locking: LR091346
Same, but left side: LR091348
(each lock actuator part number is different depending on which door and whether you have passive or nonpassive system, and whether you have dead lock or not).