Ambient Light Sensor (ALS) Repair Issues on iPhone 8 & iPhone X
Note: In this article, “original” refers to a screen manufactured by Apple. It doesn’t necessarily mean the specific screen that was originally fitted to a particular phone, unless stated otherwise.
What is the Ambient Light Sensor?
The ambient light sensor (ALS sensor) detects how bright the surrounding light is. Your iPhone uses the measurement it provides to automatically adjust the screen’s brightness in response to changing light conditions.
Why The ALS Sensor Is a Problem for iPhone 8, iPhone 8 Plus and iPhone X Screen Replacements
Automatic adjustment of the screen brightness is a feature most people take for granted, and which they may not immediately notice when it doesn’t work.
If you change the screen on an iPhone 8, iPhone 8 Plus or iPhone X still running iOS 11, then the ALS sensor won’t work any more. This isn’t just a problem with copies- it even affects original screens and third-party refurbished originals. You’d run into the same problem if you took the screen straight out of another brand-new phone and fitted it. Even if you swapped a complete assembly- including the ALS sensor- from one iPhone to another, the ALS still wouldn’t work.
During the boot process, the phone checks the connected screen to see if its serial number matches one that has been calibrated to the phone from the factory. If it doesn’t, then the ALS sensor is disabled. This only happens during boot- if you boot the phone with the screen it first came with, then hot swap it without disconnecting the battery (which we really don’t recommend!), the ALS works until the phone is rebooted.
How to Get Round the Problem
Initially, only Apple was able to fix this issue, but now it is possible to buy a programmer that can reprogram the LCD or OLED screen. Using the programmer, you read the calibration data from the customer’s previous screen and write it to the new screen. This doesn’t work on copy screens as they don’t contain an original IC to transfer it to; it only works on original (and refurbished) LCDs and OLEDs.
You can see our screen programmer in the images. (At present, we’re one of very few local companies that both fit third-party-refurbished original screens and own and use one of these- see below for why this is a much better fix than the iOS 12 update).
Another caveat is that if another company has already replaced the phone’s screen without transferring the serial/calibration data, then the serial number data on that screen won’t match- which means transferring its (incorrect) serial number data to the new screen will do you no good!
Has iOS 12 Fixed This? Yes- and No
The problem of the non-functioning ALS sensor was apparently fixed in the iOS 12 beta (released in June 2018). This meant that you no longer needed to reprogram the LCD for the ALS to function correctly. However, there was some concern as to whether this would make it into the final release.
As it turned out, it appears that the first officially-released version (iOS 12.0, released on the 17 September 2018) retains the fix. This is good news as far as it goes, but there are still some problems…
The first is Apple’s True Tone feature that alters the screen’s colour balance (rather than brightness) to match the ambient light. This wasn’t affected by the ALS issue under iOS 11… but it is disabled under iOS 12 if you’re using a replacement screen that hasn’t had the serial transferred.
The other issue is that there’s nothing stopping Apple from reverting the ALS fix at any time as part of an update. If that were to happen, the ALS (and hence automatic brightness adjustment) would stop working again on any phones whose screens had been replaced without correctly transferring the serial number.
(Since we’re one of the few local repair businesses to own a serial reprogrammer, this means that our third-party-refurbished original screen replacements won’t suffer the same risk of the ALS being disabled again, and that you can continue to enjoy True Tone with your new screen, even under iOS 12).
Another problem is that Apple can tell, without opening the phone, if the screen has been replaced by reading this data. They may, or may not, deny service based on this.
All these possibilities represent potential future problems for the industry.
Downloading the serial and calibration data from the original screen.
Transferring the data to the new screen.
