It only seems to be the Sj brand that loses the speed sensor after about 6 months although the fan is working fine.
I've also started having YaLn (Yate Loon) fans fail and completely stop.
Near the faulty fan cut the yellow wire and bare the end that goes to the plug. Then slightly bare the yellow wire (using a lighter flame to soften the plastic) on the working fan. Twist the broken fan wire around the working fan wire and cover with tape. This will make the controller show both fans at the same rpm. I think the reason the Sj fans are failing is because the tiny magnetic speed sensor stops working. Ultraflow fans seem really good but it's hard to find a low cost supply.
This is a TERRIBLE idea. The fans need to function at different speeds for air flow purposes and faking the tach signal with a fan spinning faster will cause issues. Fans are cheap, just replace the damn thing.