Door locking

Not long after we got it the sensors on the rear doors to unlock the car started to work intermittently, then stopped working. I complained about it at the time but didn’t take it in to the dealership which is miles and miles away. Over winter one of the covers fell off one of the sensors. When I took it in they told me that it had failed because the cover was missing. I told them it failed before that. No matter, they said, the prism that makes the sensor isn’t covered anyway.