There is an opto-sensor on the front right side of the tri-oval. Go into settings, tests, switch edges. Block the path between the opto led and sensor. Make sure it is correctly working each time you block it. It could be dirty, broken or bad solder joint. Try cleaning it first. Q-tip and alcohol. If that doesn't do the trick, in switch test mode, block the sensor and wiggle the wires to see if you have a loose wire or bad solder joint. I hope it's something that easy.