While attempting repairs with unofficial parts has always been risky, a new report claims that even genuine Apple replacement screens can cause sensor problems for the latest iPhones — if they’re installed by unauthorized shops. Engadget says that an ambient light sensor failure has been verified on iPhone 8, 8 Plus, and X models running multiple versions of iOS 11, and it only affects screens installed by unauthorized repairers. The same display will have “no problem” if installed by Apple or an authorized partner.

Describing the problem as “subtle,” the report suggests that the ambient light sensor will be disabled by iOS if the iPhone’s screen is swapped. It’s unclear whether this is a bug or intentional, since repair shops previously discovered that replacing the Touch ID sensor resulted in an Error 53 — failure of a security test, which Apple later disabled in an iOS 9 update. Unlike the Error 53 situation, there’s no obvious security risk in swapping a light sensor.

While fixing the problem could be as simple as updating iOS 11, independent repair shops are said to be concerned that the malfunctioning sensor is the start of something bigger: “a test-case to see how it works, to see if they can link hardware with logic boards so if they were repaired outside of the Apple network it loses functionality.” If customers believe that only Apple-authorized repairs will restore iPhones to full functionality, unauthorized shops could be run out of business.

The report notes that Apple has a special repair machine called Horizon that tells the iPhone that its new parts now…