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The Sabertooth X79 shows CPU FAN Error when POST cannot detect a fan signal. ASUS describes the message this way in its support FAQ (updated September 14, 2026): “When you see the error message ‘CPU FAN Error’ in POST, it means the system doesn’t detect the FAN.” The fan may be stopped, connected to the wrong header, spinning below the alarm threshold, or part of a liquid cooler that provides no tachometer signal.
What the Sabertooth X79 is checking
The motherboard expects a tachometer signal from the CPU cooler through its CPU_FAN header. In the Sabertooth X79 hardware monitor, CPU Fan Speed appears as an RPM value when a signal is present and as N/A when the board receives none. A fan that visibly spins can still produce no usable signal if its cable, header, control mode, or threshold is wrong.
Fix CPU FAN Error safely
- Shut the computer down. Turn off the power supply and remove AC power before touching the cooler cable. Do not continue normal operation if the CPU fan is stopped or the processor temperature is rising abnormally.
- Inspect the cooler. Check that the heatsink is firmly mounted, the fan blades turn freely, and no cable is obstructing them. A non-spinning fan, damaged lead, or failed cooler requires repair or replacement rather than a BIOS setting change.
- Connect the tachometer lead to CPU_FAN. ASUS specifies the CPU_FAN header, not CHA_FAN. With power removed, reseat the plug and verify that it is fully inserted. A cable left on CHA_FAN can leave CPU Fan Speed at N/A.
- Check the BIOS reading. Enter the firmware setup and open the Sabertooth X79 Monitor menu. Read CPU Fan Speed: an RPM number means the board is receiving a signal; N/A means it is not.
- Lower the alarm threshold only after confirming operation. If the fan works and an RPM value is visible, use the CPU-fan monitoring controls described below. Never hide an alarm from a stopped or overheating fan.
Set the CPU-fan low-speed limit
Some third-party fans rotate below the default detection point. ASUS recommends a CPU Fan Speed Low Limit of 200 RPM for a low-revolution fan that is otherwise operating. On the X79, the available choices are Ignore, 200, 300, 400, 500, and 600 RPM. The low-limit option appears when CPU Q-Fan Control is enabled.
Choose 200 RPM only when the Monitor screen shows a stable RPM value and the processor remains at a normal temperature. Selecting Ignore for an ordinary air cooler removes the warning instead of fixing the missing signal.
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Liquid-cooling setups without a CPU_FAN tachometer
Some pumps or fan controllers power the cooling system without sending a tachometer signal to CPU_FAN. ASUS documents setting Monitor → CPU FAN Speed → Ignore for this situation. Before doing so, confirm all of the following:
- The pump is powered and operating.
- The radiator fans are powered and spinning.
- The CPU temperature is normal in the BIOS monitor and during normal use.
Ignoring the reading is appropriate only when the cooler genuinely cannot provide a CPU_FAN signal. It disables a protective warning, so a later pump or fan failure may no longer stop boot with an alert.
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Use the observation to choose the next action
| BIOS or physical observation | Likely interpretation | Next action |
|---|---|---|
| Fan does not spin | Fan, power lead, or cooler failure | Power off and repair or replace the cooler or fan. |
| Fan spins and an RPM is visible, but POST still warns | Low-limit or control-mode mismatch | Enable CPU Q-Fan as appropriate and try the 200 RPM low limit. |
| CPU_FAN shows N/A and the cable is in CHA_FAN | Wrong header | Move the cable to CPU_FAN with power removed. |
| Liquid cooler has no CPU_FAN tachometer | Expected missing signal | Verify pump, radiator fans, and temperatures, then set CPU FAN Speed to Ignore. |
| All physical and BIOS checks pass but POST still warns | Possible BIOS or motherboard-sensing issue | Update the BIOS and contact ASUS support if the warning remains. |
When a replacement cooler is necessary
The Sabertooth X79 uses the LGA2011 platform. If inspection finds a failed or incompatible cooler, select an LGA2011-compatible CPU cooler fan only after checking the exact processor, mounting bracket, cooler height, case clearance, and whether the replacement uses a 3-pin or 4-pin connector. A generic fan is not guaranteed to fit or to provide the signal this motherboard expects.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.BIOS update and support
ASUS recommends updating the Sabertooth X79 to the latest BIOS when the warning persists after the wiring, RPM, and cooling checks. No current BIOS version number is established here, so use ASUS’s support page for the board’s currently published files and instructions. If the error continues after the update, ASUS advises contacting customer support; persistent N/A readings can indicate a motherboard header or sensing fault.
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