Sequre S99 soldering irons with PCB version 1.3 require a modification. This was documented by Sequre on their website and I made a video on how to perform the mod.
https://www.youtube.com/watch?v=Jsqc4lFnLAw
In Jan 2026 I was informed by a viewer that Sequre had removed the bulletin from their website. I enquire with Sequre and they confirmed they removed it as they feel it's not an issue anymore.
I've recovered the relevant information using WayBack Machine and I am adding it here for future reference. This only applies to PCB version 1.3. Please do contact Sequre if you're unable to solder that component yourself, they might be able to help you.
WayBackMachine page: https://web.archive.org/web/20240519173242/https://sequremall.com/blogs/sequre-product-firmware-upgrade/s99-v1-07-firmware-upgrade