If inside the vehicle, the doors must be unlocked to operate the back door by using the cockpit button. You must also be in park. If you are at the back door with fob in pocket, press and hold the door release and it should open. Note the "dimple" to the right of the door release. Push it and it will lock all the doors just like pushing one of the handle buttons on the other doors, but only if the back door is closed.
The Lexus keyless entry is one of my favorite features and will spoil you to all other vehicles. I just got my 5k service completed yesterday and this is my third LX.
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