Batch 1 user here. Framework Support has confirmed to me today that this is a known issue:
We have confirmed that this is a known issue.. At this time, there is no need to perform any troubleshooting steps, as our we actively working on a software solution. We will provide an update as soon as the fix becomes available.
The fan detection error is only shown when the firmware option [Advanced] > [Boot performance mode] is set to “Max battery”. If you change it to “Max battery”, the detection error only shows up after EC reset (approx. 15 seconds after power off). Vice versa. Setting it to either of the two remaining options mitigates the issue.
I’ve been in contact with Framework support in the past month. I’ve even gone through the process of “switching it off and on again”, exchanging components, and even replacing the main board. None of these measures work, as it’s fundamentally not a hardware issue.
PS: unfortunately I upgraded the firmware on first boot so I don’t know whether firmware version 3.0.4 introduces the issue or not.