I built a new PC with a Ryzen 7 7800X3D and an ASUS TUF Gaming B650E-PLUS WIFI motherboard. The first boot was normal, but on later boots the BIOS began reporting that the CPU fan was not running. The cooler fans briefly stop immediately after pressing the power button, then start spinning normally and continue running. By the time the warning appears, BIOS shows the CPU_FAN header at around 750 RPM. The CPU Fan Speed Lower Limit is currently set to 200 RPM. Temperatures are normal, but I would like to fix the warning rather than simply disable it. Could the brief startup pause or the fan-speed threshold be causing this, and is there any risk of the CPU overheating if the fan actually fails?
3 Answers
First, confirm that the cooler is connected to the motherboard header labeled CPU_FAN rather than a nearby chassis-fan header. If the header is correct and BIOS is detecting roughly 750 RPM, the connection is probably fine. Modern CPUs also have thermal protection and will throttle or shut down if they get dangerously hot, so a fan warning will not normally result in the processor burning up.
Check the CPU fan curve as well. If the curve commands 0% or an extremely low duty cycle at startup, the fan may stop briefly and trigger the BIOS check. Set a small minimum speed or a low-temperature point that keeps the fan spinning, then save the settings and test a few cold boots. The brief stop may also be normal fan-stop behavior, but calibration and a modest minimum speed are safer than ignoring the alert.
Run ASUS's fan calibration or minimum/maximum speed test in the Q-Fan or hardware-monitoring section of BIOS. Some fans do not start reliably at very low PWM settings, and the board may check the RPM before the fan has fully spun up. After calibration, try increasing the CPU_FAN lower limit from 200 RPM to around 500–600 RPM, or slightly above the fan's reliable minimum. Make sure the warning disappears without setting the header to Ignore.
A lower limit around 200 RPM may be below this fan's actual startup speed. Raising it after calibration should help, but avoid setting it higher than the fan can maintain when the CPU is cool.
The cooler is definitely connected to CPU_FAN, and BIOS detects it at around 750 RPM. It only pauses briefly during startup, then runs normally by the time the warning is displayed.