I bought a used key fob from a commercial fob retailer on eBay, and it is the same model as the original fob that came with the car. I took it to a Lexus dealership to get programmed, and they tried to program it, but were unable to and told me that it cannot be programmed. When I asked why (is it defective, incompatible, some other reason), the technician couldn't answer the question confidently. First he told me that once a fob is programmed for a car, it cannot be reprogrammed for another. If that's true, I responded, why do they sell used ones on eBay? Then he basically told me that you have a 50/50 chance of making a used one work.

This does not make sense to me. Either used fobs can be reprogrammed for another vehicle or they cannot. I don't understand how there can be some magical variable that determines whether a compatible model fob will work with a given car.

Can someone shed some light on this? Thanks!