I have been receiving this message on booting my Dimension 8400 "CPU fan failure.  Press F1 to resume.  Press F2 to setup"

My fan seems to be working fine.

I checked the Dell Troubleshooting site and found a post for exactly this error.  It said it's a false error and I should update the bios.  I followed the instructions to download and update the bios yesterday (updated bios is A09) and thought my problem was fixed.  But this morning I got the same error message again.

Any other ideas?  I think it's a false error as the fan seems to be spinning.  But it's annoying.