Apex locator in retreatment cases:

-    Electronic apex locators may give misread for working length when gutta-percha is initially being removed.
-    Due to that file beging covered with chloropercha that may affect its conductivity.

Removal of overextended GP:

-    Inserting a new hedstrom 0.5 to 1 mm beyond the apical constriction, using a gentle clockwise rotation then slowly and and firmly withdrawn with no rotation, removing  the overextended material.
-    Avoid excessive force the instrument.
-    Not softened the overextended GP with solvent.
-    remove the overextended gutta percha by barbed broach or by H file
-    after remove of gutta percha, heat the tip of  instrument and insert to catch the gutta percha and removed suddenly.