The new Fire TV interface is currently rolling out to most Fire TV models via the latest operating system update. While there has never been an official way to roll back a Fire TV software update, Amazon seems to especially want the new interface update to be a one-way path. It seems as though Amazon has decided to blow an eFuse, at least with the Fire TV Stick 4K, when the software update with the new interface is installed. This makes it impossible to downgrade back to the old interface, even if the device is rooted.
XDA forum member July IGHOR installed Fire OS v18.104.22.168 on their rooted Fire TV Stick 4K, which upgrades the device to the new interface, and found that the device would no longer boot if downgraded. This is unusual because, while a stock Fire TV cannot be downgraded, it’s normally trivial for a rooted Fire TV with a custom recovery installed to downgrade its operating system. Upgrading back up to v22.214.171.124 presumably recovered the Fire TV Stick to working order, but downgrading to an older version was no longer an option.
Rbox, a developer who has been active among the Fire TV modding community for years, has determined that Amazon has chosen to blow an eFuse with Fire OS v126.96.36.199 on the Fire TV Stick 4K to prevent downgrading. He says a script was added in a previous Fire TV software update that did nothing, but that script was activated in the latest software update to blow an eFuse.
An eFuse, or electronic fuse, is a microscopic fuse put into a computer chip that can be triggered to blow in order to permanently change the circuitry of a chip. While eFuses have other uses, they are most commonly used by hardware manufacturers to prevent older software from running on the device. The older software expects a certain number of eFuses to be intact when the device boots, so blowing an eFuse makes older software incompatible with the device.
Amazon has blown eFuses on Fire TV devices in the past as a way to seemingly prevent newer devices from taking advantage of older software exploits that allowed the device to be rooted or allowed the bootloader to be unlocked. This latest blown eFuse seems to be a way to keep devices on the new Fire TV interface, but we’ll likely never know if that’s actually the reason. Regardless of the motivation, the fact is that a Fire TV Stick 4K that updates to the stock version of Fire OS 188.8.131.52 will not be able to downgrade, even if it is later rooted.
The good news for rooted Fire TV Stick 4K owners is that Rbox has already released a pre-rooted version of Fire OS 184.108.40.206 for the Fire TV Stick 4K that does not blow the eFuse. As long as the device has never installed the stock 220.127.116.11 update, you can install the pre-rooted version of 18.104.22.168 and still downgrade if you want. Please be sure to donate to Rbox to support him and thank him for continuing to keep all of our rooted devices updated.