Apple iPhone 8 & iPhone X ALS Sensor Issues for Repair

The ALS Sensor- What is It?

ALS is short for “ambient light sensor”, and it detects the brightness of the surrounding light. The iPhone uses readings from it to automatically change the screen’s brightness in response to the current light conditions.

What Does It Have To Do With iPhone 8, iPhone 8 Plus and iPhone X Replacement Screens?

The automatic adjustment of the screen’s brightness level is something most people won’t notice in normal use- in fact, many won’t be aware of it at all. There’s also a good chance they won’t immediately notice when it stops working (although they’ll probably find their iPhone less comfortable to use).

Here’s the problem. If you replace the screen on an iPhone 8, 8 Plus or X and it’s still running iOS 11, then the ALS sensor won’t work any longer. It makes no difference whether you replace the previous screen with a copy or original one… not even if you replace the complete assembly including ALS sensor with one from another iPhone. The ALS still won’t function- end of story.

The reason is that during the boot process, iOS checks the serial number on the current screen to see if it matches one that was calibrated to the phone at the factory. If this isn’t the case, then the ALS sensor is disabled. The check only happens during boot- if you boot using the original screen, then replace it while the phone’s still powered-up and running, the ALS continues to work until the phone is next rebooted. (Disclaimer; you probably don’t want to test this yourself, since changing the screen- or carrying out any repair- while the battery is connected stands a very good chance of frying the phone!)

Working Round the Issue

At first, it was only Apple that was able to get round this problem. However, you’re now able to buy a device that’s able to reprogram the LCD or OLED screen.

The programmer reads the calibration and serial data from the old screen and writes it to the new one. (This only works with original screens, not copies, since those don’t have an original IC).

(Our programmer is shown in the images- we’re one of very few local companies that own one, and we explain below why this makes for a more reliable repair, even after the iOS 12 “fix” update).

One other issue is that if the screen has been replaced on a previous occasion without the original serial/calibration data having been correctly transferred, then copying the data off that won’t do you much good- it didn’t match in the first place.

Problem Solved with iOS 12? Possibly- Possibly Not.

Although it seemed the non-functioning ALS sensor issue had been fixed with the beta version of iOS 12 in June 2018 (meaning the replacement LCD no longer needed to be reprogrammed for the ALS to continue working), there were worries whether this would make it into the final release. It turns out that- yes- the final iOS 12.0 (which was released in mid-September) kept it in, which is good… as far as it goes.

However, there’s another problem. Apple’s True Tone feature– that automatically alters the colour balance (rather than brightness) to match the ambient light- wasn’t affected by the serial/ALS issue under iOS 11, but it won’t function under iOS 12 if the serial hasn’t been transferred to the new screen.

As well as that, there’s nothing to stop Apple from changing the ALS behaviour back again as part of an update. Were this to happen, countless phones whose replacement screens hadn’t been updated to match the original serial would once again see their ALS sensors- and automatic brightness adjustment- stop working.

(Because we’re one of very few businesses in the local area who own a serial reprogrammer, this means there’s far less risk that your ALS will be re-disabled in future with our original screen replacements, and also that True Tone will continue to work on your new screen, even after updating to iOS 12).

It also highlights that Apple can tell- without having opened the phone- whether a replacement screen has been installed, simply by reading this data. They might- or might not- deny service based on that.

Both represent possible issues for the independent repair industry in the future.

Transferring the calibration and serial data from the original screen into the programmer.

Moving the data over to the new screen.