In my experience with the ultrasonics it is very rare that they go bad. I usually find a bad wire in the cat track on the gantry or a wire in the boom that attaches to the wall. It is good to have a spare ultrasonic to hook up to see if the wires are bad somewhere before installing one. I always check in the boom where the wires comes in and goes out. That is usually the place for a rub through. THis week I found a bad wire in the cable that showed no signs of damage to the insulation but the wire was corroded anyway.