I have a humminbird 959 with latest firmware and had the problem when it was set to Fresh water and salt water shallow. Would read the bottom as being 1.6 ft, when switching to deep salt the reading would work again. I called Humminbird and they said send it in for evaluation ($300) or try using a buddies components (don't have buddy that uses humminbird) or try changing the probe ($125). I elected to get a new probe (cheapest option) and the unit works great again.