Rear door lock makes trrrr sound — lock mechanism replacement

Inside the lock mechanism are plastic and metal gears and a stepper motor. When teeth are missing from the plastic gears, the stepper motor spins but the lock does not open — resulting in a fast chattering trrrr sound.

The lock mechanism must be replaced entirely. Just oiling does not help if the plastic gear wheels are worn. The fault does not repair itself and gets worse over time.