My Ryzen 7 9700X used to show about 30°C in the BIOS with an Aorus Elite B650 AX V2 and a Thermalright Phantom Spirit 120SE. After replacing the motherboard with an MSI MAG B850 Tomahawk Max WiFi, I also upgraded the power supply from a Corsair RM750x to an RM850x for a future GPU upgrade. The new motherboard reports roughly 60°C for the CPU in the BIOS, even after updating the BIOS, disabling PBO, tightening the cooler, and trying an undervolt. A Fedora live environment shows CPU0 around 60°C while the other cores sit near 45°C at idle. I cleaned off the original Thermal Grizzly paste and reapplied Arctic MX-4. Is this temperature difference normal, or could there be a problem with the motherboard, cooler installation, or temperature reporting?
3 Answers
The PSU is very unlikely to be responsible for CPU temperature. Different motherboards can report temperatures differently, and the BIOS may run the CPU or leave the fans stopped until a temperature threshold is reached. Check the readings with more than one monitoring tool in the operating system, then compare idle temperatures with temperatures during a known CPU load. Also verify whether the CPU fans are configured to stop or run very slowly around 60°C.
A single core being around 60°C while the others are closer to 45°C does not automatically indicate a fault. Ryzen chips can have one active core doing background work, and temperature readings can jump quickly. Still, remove and reinstall the cooler if you are unsure: clean both surfaces, apply fresh paste, and make sure the mounting pressure is even. When you remove it, check whether the paste spread across the CPU heat spreader; a well-covered, even imprint suggests the cooler was making good contact.
The old paste was fully cleaned off before I applied MX-4. I’m planning to redo the cooler installation in a more comfortable workspace so I can inspect the spread and mounting.
The motherboard is the main new component that could explain a persistent difference, especially if the temperature is high only in its firmware screen. BIOS temperatures are not always comparable between boards. Check actual idle and load temperatures in the installed operating system, confirm that the pump or fan settings are correct, and see whether the GPU and other components are at normal temperatures. If several tools agree that the CPU stays unusually hot after a careful cooler reinstall, the motherboard would be more suspicious than the power supply.
Before returning anything, I’d repeat the test after reinstalling the cooler and checking the fan curve. If the cooler imprint looks normal but the new board still reports much higher temperatures in both firmware and the operating system, then contacting the board manufacturer or exchanging it would be reasonable.
I don't remember setting a fan-stop threshold, but that may be how the board is behaving by default. I'll check the fan curve and compare several monitoring tools.