please report back if that solves the issue, as I still think the solution is not in the BIOS...

I agree @elisw I think this may only help with non-PWM fans, or PWM Fans that do not have their own PWM controller but rely on the boards control. I'm only changing the BIOS min duty setting below what Asus locked as the default min.

@Lost_N_BIOS
Hello, thanks for modding. It seems like that modding doesn't work ! The flashing process is successful, I've already unlock with your step in post #7 (thanks for help at variable step) & FPTw.exe does not showing any error when flash with "FPTw.exe -bios -f biosregm.bin". All is good in this side !
The Chassis Duty Fan Min. is always at 60%, when I tried to put lower than 60% it show "Invalid Input Range" like in stock BIOS version ! I tried in "Silent", "Standard", "Turbo" & "Manual" mode !
And, Yes I misleading the CPU Fan Min. is not 40%, is at 20% !

Maybe another "60%" Chassis Fan Min. section to mod ? For information I'm under "Manual" mode !

Another question, Is a flashing modded BIOS will affect something in motherboard (Software), like SERIAL Number or whatever else ?

For information there is (EXACTLY) my CHASSIS FAN : https://www.ebay.com/itm/5-Pcs-FG-PWM-12...Z3zXR:rk:1:pf:1
connected to a fan hub (EXACTLY) powered by Sata : https://www.ebay.com/itm/10x-4-pin-PC-CP...Zdx6A:rk:1:pf:1
which is connected to the Chassis Fan Header on motherboard.

PS : Only one fan connected to the hub, not the five ! (Because I have 5 fans package on my computer)

Yes, flashing mod BIOS can affect your serial, UUID etc, don't flash stock BIOS mod with FPT, only FPT dump then mod then flash. We're already doing correctly to preserve your details, that is why I asked for FPT dump from you first (Since no USB Flashback on this model)

So chassis fan still said 60% before you touched it? If yes, thanks, good to know, now to test next change on top of one I already set (this is stored in a few locations) I will make new test BIOS for you tonight, please wait, I bet we can get it!

For testing, once we get visible change in BIOS, you need to connect fan directly to header, to be sure changes are being applied. Otherwise, fan hub will be powering fan speed not the board.

Yes, before i touch it, he was already at 60% before i change to go lower (but nothing) !
Yes I've already put ONE fan only on the "Chassis fan" header (without the hub).

I believe you, only asking to confirm. This setting is held in a few places, I assumed the one I changed would over-ride NVRAM ones, but I see it doesn't, so we change there next!

* Edit - here is next test. There is still many more places to change this in NVRAM next if this one fails, this one is setup module change, first was AMITSE change.
I figured we should try setup module change first anyway since AMIBCP/AMITSE-SetupData change didn't get used (it's usually either this, or setup used)
http://s000.tinyupload.com/index.php?fil...519224826807414

If this fails, then I'm sure NVRAM changes would get it next time around!

The second mod not working too ! Proceed to the next change ! Already at 60% before touched it too !

How are you flashing? Please show me your AFU commands or boxes checked if using AFU.
It may simply be the change needs to be done in NVRAM, but I find it curious neither of the two changes we tested already reflected into the BIOS because one of them does in AMIBCP, which usually means it will be used (but not always).
So that's why I wondered how exactly you're flashing.

@Lost_N_BIOS I'm flashing like you tell me to do with the following command : FPTw.exe -bios -f biosregm.bin
I'm run CMD with administrator right on FPTw-V11 folder and running "FPTw.exe -bios -f biosregm.bin"
It seems to flash successfully.

Sorry for bad picture quality.

OK, thanks I forgot we were doing that and didn't look back when I asked. That's best way, and this way we know it's all going in, so thanks for confirming
Here is new test, changed at three locations in NVRAM - please test both BIOS and let me know outcome of each - http://s000.tinyupload.com/index.php?fil...200713811736274
And check setting, does it show 60, or 20, and then check actual fan speed, do you think it's lower despite if BIOS says 60 or not? I think it should show 20 in BIOS now, but just in case it doesn't, please consider actual fan speed in person too.

No, no BIOS images needed, thanks.

@Lost_N_BIOS Sorry but nothing changed in BIOS for BIOSREGM3.bin & BIOSREGM3-2.bin. Always at 60% in BIOS before change and after change (Invalid Input Range).
Edit : I always check different profile for "Chassis fan" to test any combination possible.

Thanks you for your hardwork for help me. I really appreciate.

I didn't think you'd be able to change it yet in BIOS, leave at default no matter what is shown. So, no change to actual fan speed in person with either?
For this, you have to use "Custom", otherwise it's some other preset value vs the one we're trying to change.

I will look at it more tonight!

I've opened my PC case to check fan change but nothing change with all your 4 modded BIOS. So, no change to actual fan speed. I test all presets to finish with "Custom" presets. Custom presets don't want to go below 60% (Invalid Input Range) before and after change.

But don't worry, if nothing works we have trying to do. Thank's again for help. This is not the success the best part is the helping. (Sorry for bad "English")

Edit : I can only change speed after "Windows 10" boot, with Speedfan. But in "speedfan", I need to manually decrease the value from 100% to 0% (at each boot) to calm down the fan (from 2400 RPM to 1200 RPM).

Can't you simply connect "-" or black wire of your fan, into 5V pin on molex plug ?