You would really need to post your mobo make and model as the only fan that should cause an error message is the CPU fan. But it is possible that the fan settings are set to low in the BIOS,you can,alter fan error notification settings in the BIOS.
It hasn't failed, it just takes a little longer to reach the PC's disired rpm.
It isn't the CPU, that has it's own sensor.
Mobo: ASUSTek P8H61-M LE/USB3 Rev x.0x
BIOS: American Megatrends Inc.
The Chassis/Case is a 'Cooler Master'
I've looked in the BIOS, it's well good at scare mongering. By showing that a chassis fan - in red - is taking to long to build up its revs. But it doesn't say which Chassis fan is slow to pick up!? I guess that would be to helpfull.
It only has to say something along the lines of...
As *Fruit Bat /\0/* said unplug one then boot if you get the error you know which one it is and you can alter the settings. You will find the settings in advanced mode in the monitor section of the BIOS. Chapter 2 section 6 part 2 of the manual.