https://en.wikipedia.org/wiki/E-meter

Scientologists claim that in the hands of a trained operator, the meter can indicate whether a person has been relieved from the spiritual impediment of past experiences.[42] In accordance with a 1974 federal court order, the Church of Scientology asserts that the E-meter is intended for use only in church-sanctioned auditing sessions; it is not a curative or medical device.[43] The E-meters used by the Church were previously manufactured by Scientologists at their Gold Base facility,[44] but now are manufactured in Hong Kong and Taiwan.[44]

New religious movement scholar Douglas Cowan writes that Scientologists cannot progress along the Bridge to Total Freedom without an E-meter, and that Hubbard even told Scientologists to buy two E-meters, in the event that one of them fails to operate.[27] According to anthropologist Roy Rappaport, the E-meter is a ritual object, an object that "stand[s] indexically for something intangible".[50]

They don't want their follower to repair the device, they want them to keep buying them. Also, they probably afraid that some of their follower would mod the meter to output high reading to clear the audit. lmao.