About a month ago I encountered the following error on my iPhone 5S:

Unable to activate Touch ID on this iPhone

And sure enough, the Touch ID to log in was not working. When I logged in using my passcode, I went into the Touch ID settings in the Settings app, and saw that Touch ID was disabled. When I enabled it, it briefly popped up the setup screen to enrol fingerprints but then there was a further error window. After dismissing this window, I saw that Touch ID was  disabled again.

The other symptom I noticed on this was that it seemed to occur after I’d charged the phone and it was slightly warm. Once or twice I was able to get Touch ID working again after I powered off the phone and started it up. There was no real consistency in this.

The Investigation

Thinking it may be a potential thermal expansion issue, I opened up the phone. After lifting up the front display glass/LCD assembly, I noticed that the Touch ID / home button flex retainer bracket/clip was slightly misaligned but still in place. The retainer is a tiny metal clip that holds the flex cable from the Touch ID sensor/home button assembly.

I didn’t even think anything of it, since the home button worked, but I started getting a little suspicious. Perhaps the heat of the charging caused some expansion, causing just enough expansion to cause contacts to be broken, and resulting in this intermittent behaviour. And why was the clip slightly out of place? Maybe from dropping the phone, who knows…

I popped the clip in with a satisfying click and fired up the phone, hoping for the best. Sure enough, problem solved. It has been fine for a month now.

How to Fix

It’s an easy fix with the right tools (thanks, iFixit). There are two screws next to the Lightning connector on the bottom of the phone that have to be removed and the front glass assembly will lift off. The main trick is just being careful lifting up the front glass carefully from the bottom.

It’s a tight and frustrating to separate the two parts, and lifting the glass too far (about 2″) may stretch and damage the Touch ID flex.

No other parts need to be removed to fix this, so it’s easy.

Instructions on home button removal on iFixit are here:

https://www.ifixit.com/Guide/iPhone+5s+Home+Button+Replacement/24983

You basically go to step 9, ensure the metal bracket is securely in place, then put it all back together.