It does what is called "null' when it goes over iron, null is the loss of threshold... When coil passes over piece of iron the detector will go silent, you lose the threshold sound..... In six years I have never seen it be wrong once. The best part is it will still sound off when gold , silver or plat is there with the iron..... When you get a null, always sweep the target again from a 90 degree different direction, if there is a good target there it will sound off. I found a gold ring this way, when I recovered the gold ring it was in the same scoop as the piece of iron...