Apple's latest watchOS update, software version 7.3.1, includes a very important fix---solving a bug that stops some Apple Watch Series 5 and Apple Watch SE devices from charging when they enter Power Reserve mode.

The company has also launched a new free repair program for affected devices that are trapped in this low-power mode, and unable to get out of it. This bug essentially "bricks" their Apple Watch, making it no longer functional.

The Power Reserve Mode Bug

Power Reserve mode is initiated when an Apple Watch is low on battery. It stops your Apple Watch communicating with your iPhone. It also stops users from accessing other features on the Apple Watch. To get out of Power Reserve mode, users need to charge their Apple Watch.

On a support page about the bug and repair program, dated February 15, Apple begins by noting:

A very small number of customers with Apple Watch Series 5 or Apple Watch SE models running watchOS 7.2 or 7.3 have experienced an issue with their watch not charging after it enters Power Reserve.

To find out whether you have an affected Apple Watch model, Apple recommends placing your Watch on a charger and waiting at least 30 minutes. If it will not charge, contact Apple Support and they will arrange for a free mail-in repair.

Watches will be examined to check they are eligible and, provided that they are, the repair will be carried out free of charge, and the repaired Watch returned to its owner.

It is not clear exactly how many Apple Watches are seriously affected by the bug. However, it seems that the problem only affects a subset of Apple Watch Series 5 or Apple Watch SE models, and no other models.

How to Upgrade to WatchOS 7.3.1

For those who have not experienced the bug, it's still worth upgrading to the latest watchOS version just to protect yourself against it potentially happening.

The importance of the bug fix is indicated by the fact that Apple immediately pushed out watchOS 7.3.1 to all users. This is in contrast to the way that it normally issues software updates by making them available initially to developers for beta testing. Apple released watchOS 7.3 in late January.

Users can download watchOS 7.3.1 using the Apple Watch app on iPhone. To do this, they must go to General > Software Update. Your Apple Watch needs to have at least 50 percent battery in order to update. It must also be in close proximity to your iPhone.