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Before you dive in: 1. Examine the underside of your sink for unsuspected leaks
Run water through the drain and check for any cracks or leaks that need to be repaired before you start your job. Also test the old P-trap by pushing a thumb on the underside as shown. If it’s soft, it’s high time to change it out—with plastic if it’s not in the public eye or with chrome if it’s exposed.