The magnet is the part that moves back and forth with the diaphragm.  It holds the diaphragm secure and vice a versa.  Typically the magnet is very long lasting.  However if you run the air pump with bad diaphragms for extended periods you could wear down the magnet.  If you open the motor for repair and you find dust or small metal particles you may need to replace the magnet.  Always inspect the magnet for wear and tear when you replace your diaphragms.  They are very easy to replace.  Call us if you need help.