L512 fan always at 100% speed- any ideas or how can I downgrade the BIOS?

Got a nice cheap L512 with only one issue: Soon, like 5 sec after I press the powerbutton, the system fan is at 100% and remains there. Seems like the BIOS doesnt let him change the speed. The result are perfect temps, but its annoying. I tried these steps to find the issue:

renewed the thermal paste, no dust in the fan, put a bit of oil in it (with caution)

searched for BIOS settings to change the speed, changed the CMOS battery and made a BIOS factory reset

tried to remove the CMOS battery for a bit, nothing changed

tried TPFancontroll, any changes I made there hadnt any effect

tried the same fan from my SL510, perfectly silent from that machine, connected to the L512 as loud as the original one, the fan itself should not be broken

tried to update or change the BIOS with onboard utility and CD, wasnt successfull, newest version is present, wont let me update it

now my question is: How can I downgrade the BIOS for this model? I know how to do it with e.g. my X230t, but havent found anything for this less poplar device. I want to renew the BIOS totally, not only resetting it to factory defaults. Hope this solves the crazy issue

Would be awesome if anyboy could help. Im very much open for other suggestions to silence the fan a bit. My last idea is to live with this defect, build a resistor or potentiometer in it to silence it myself:)

Edit: I think the whole BIOS is corrupted, the device doesnt power off when I hit shutdown, the screen goes black, the status lights and the annoying fan remain on, the power button blinks. A replacement mainboard is maybe not worth it. I cant reprogram the BIOS chip with another laptop, havent done this until now and havent got the needed tools