What happen if the ghs become 1.. is it still useable? Or need technician to fix it
I think after 1 ghs, your miner will still work some time then it will stop if you will not fix them by using a technician.
I believe it runs until 4.99% health (where it would round down to 0 GH/s). But it's a lot better to keep it repaired.
My 10ghs is already down.. It dont even drop to 7..
Edit: I found "repair" and "hire technician" option .. the game goes on .. (stupid me

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It shouldn't die straight off. It should go down at like 3-5% per day, so drop by 1GH/s every 2-4 days, until 0. I'd repair long before then, though.
It was around 18 days without any action. I hire technician for one hour and turn off everything for one hour for repair - now is mining for 100%.