Upgrading or downgrading the firmware or factory reset and replace ribbon wire(18 pins)doesn't work you have a faulty temp sensor that is why it gives you 15 temp.

Can you try to increase the fan speed and lower the frequency if you don't know how to do this you can check this method from here https://bitcointalk.org/index.php?topic=2690140.0 you can find the frequency in the same tab.

If still doesn't work you have a hardware issue the only solution is to replace the temp sensor from the hashboard.