Hi Mike,
I had purchased a replacement position encoding strip for the back massage unit, removed the module from the back to install it, and it seemed to work properly after I reassembled it. I didn't see anything wrong as the old encoder strip was still in good shape, and didn't need to do any electronic repair. My guess is some lint was blocking the optical sensor so it could not read the encoder strip.
I did replace the belt as well - it was pretty worn and seemed to slip easily, but I don't think that was the issue.
I should also note the EP30004 appears to be an economized EP30006, so the directions provided in the manual I found will also work with your model.