I recently upgraded a prebuilt system from an Intel Core i5-10600K to a Ryzen 7 5800XT, along with a ROG Strix B550-F Gaming WiFi II motherboard. The computer worked normally for about a week, but now the CPU quickly climbs past 100°C and shuts down even while sitting in BIOS. I reapplied the thermal paste using a pea-sized amount in the center, and the AIO cooler is connected to the CPU_FAN header. The cooler seems to move slightly even after tightening the mounting caps. When I gently pressed the cooler down against the CPU, the temperature briefly dropped from around 79°C to 49°C, then rose again after I released it. The AIO is several years old and was previously used without problems. What should I check first?
4 Answers
The cooler should not shift at all. That temperature drop when you press it down strongly suggests the cold plate is not making proper contact with the CPU. Recheck the AM4 mounting hardware, the standoffs, and the cooler bracket, and tighten the screws evenly in an X pattern. Make sure the correct mounting hardware for the new motherboard is being used.
Double-check that there is no protective plastic film left on the cooler plate and that the radiator and pump are installed in a way that keeps air from collecting in the pump. A top-mounted radiator is generally safest; with a front-mounted radiator, the tubes should normally be at the bottom when possible. The pump should be connected to the motherboard's AIO_PUMP header if available, though using CPU_FAN can still work if the pump is receiving full power and reporting its speed.
Pressing the cooler down made the temperature fall almost immediately, so I’m going to revisit the mounting pressure and the double-ended standoffs rather than keep running it. The mounting caps feel tight, but the cooler still has a little play, which probably means something underneath is installed incorrectly.
The AIO pump may have failed, especially since it is more than five years old. In BIOS, check whether the pump header reports a sensible RPM, and feel or listen for a slight vibration from the pump. Also confirm that the pump is powered and that the BIOS is not stopping it because of an incorrect fan-control setting. If the pump is running but the CPU still overheats, the cooler itself may need replacement.

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