There are several reasons why it might be time to replace the door lock. Since a properly locked door is often your first line of defense against any kind of unwanted threat, it's important that you actively take care of it, even when it's time to replace it.
While you may view this step as a waste of money, it is far less than what you would incur if your key failure resulted in theft. Here are great times to change your locks from a reputable service provider like All Things Locks:
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Old door lock
As each house ages, so do its doors. Any wear and tear your keys experience can be a serious weak point for your home security. It's easier to pick up and break old worn-out keys. In this case, you will have to replace the key.
If you are renting out property or rooms in your home, it is a good precaution to change the door locks whenever the previous tenants move out. This ensures that even if the key is returned, there is no chance that you will still be logged in if you have made a backup. You will have peace of mind that if a breakthrough happens, they don't.
If you are the victim of a robbery, you should get a replacement door lock. Also, someone can have access to your house keys without your knowledge, another reason why you should change your door locks.