JARblue

If it turns both ways with the normal resistance then it's probably the right key. If the key turns loosely, it could be something failed with the internal locking mechanism and it will be difficult to determine whether the key is correct or not. Assuming a working lock, normal operation would allow the wrong key (from another Z fob) to insert fine but it won't turn either direction.

My uneducated guess is the previous owner didn't replace anything and the lock is simply broken. I would remove the door handle and take it to a locksmith first to see what they think. The dealer should be able to replace it but it could be expensive. Seems like you could use the same fob if you changed out the locks with different keys but I'm not sure that will be cheaper if you can't find used sets.

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