The door locks will get sticky, and their pins will get a bit loose with wear and tear.  This is why a lot of Mazda keys will work on multiple cars with worn locks!  A shot of silicon spray will help keep the lock mechanisms lubricated.  Those in colder climates can use the tried-and-true dry graphite dust if liquid lubricants have a possibility of freezing.  Damaged locks that do not work totally will need to be replaced; don't fear of having multiple keys for different doors - these lock cylinders can usually be "rekeyed" for your key (code).

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