Check the DB9 connector where it connects to the control box. Make sure there are no burnt pins on the connector on either side. Then put the connector back on without tightening the screws. The screws are known to be too long and push the connector out slightly, causing issues with the stepper motors.
Try to clean the pins as well as you can and then plug it back in and out a few times to clean off the female side. Then plug it back in and make sure its fully seated without tightening the screws all the way. It wouldn’t hurt to put some dielectric grease on the connection, if you happen to have any.
If it still doesn’t work you should contact Langmuir support for replacement parts.