It does not seem like the motherboard is sensing the fan s properly.
Dell optiplex cpu fan error.
Turn off the pc.
Use a can of compressed air to remove dust or debris from the air vents dust filters if any and cooling fan fins.
Previous rear fan failure press f1 to continue.
I m a big fan of dell a dell user and not an employee.
Air vents may be located in various places depending on the pc model.
There is a sensor for temperature on the board and on the power supply.
If you have an optiplex stating its model number and size speeds troubleshooting.
New optiplex 390 fan failure message we believe it is a sensor error.
All of the fans inside the system are running normally.
Better you go to the dell service center with your purchase bill.
We ve swapped the cpu fan and the front case fan from other working optiplex 3010 s and the message still appears.
Cleaning the air vents on your dell pc.
Check the air vents on the pc for dust or debris.
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Refer to the user manual of your dell pc.