Dale, first check the proximity switch at the bottom of the mast. With the mast lowered the switch should show a green light. When the mast raises and the metal bar extends beyond the switch the light should go out. No light switch is bad. Also check the height encoder at the top of the mast. Conventional means can be used to diagnose the encoders as follows. Using a voltmeter, determine if 5 volts and battery negative are going to each encoder by back-probing the connector. If good, then connect to one output channel and battery negative, and rotate the encoder very slowly to see if the input goes on/off/on (0 then 5 volts) when the encoder is rotated slowly. Check the other bit also. If they are both outputting, the encoder is OK. Let me know if you need additional help, e-mail in my profile.