Locking Mechanism Not Working

Thanks for posting. By locking mechanism, do you mean the mortise lock within the door? If you remove the handle set from the door and try manually 'throwing' the lock with a slotted screwdriver, does it fail to budge? If so, then yes, it does need to be replaced. I believe you have the 82-265. Review the diagram below to confirm it matches what you have. For a compatible keeper, see the 82-249.

Alternate image for 82-265