1. Problem in operation: if the lock is difficult to open or close or the key does not turn easily. 2. Damaged: If there is a crack, fracture or other damage to the lock body. 3. Lost key: If the lock key is lost and security is important. 4. Theft or attempted theft: After any attempt to open the lock by force, it is better to change the lock. 5. Obsolescence: if the lock is old and no longer reliable.