Ryzen 5 3600 Idles at 90–95°C and Shuts Down

The most likely issue is poor heat transfer between the CPU and cooler. Check that any protective plastic was removed from the cooler’s contact plate, that the correct AM4 mounting hardware is being used, and that the cooler is firmly making even contact with the CPU. While the system is showing 95°C, carefully feel the heatsink or radiator. If it stays cool, heat is not reaching the cooler properly. If it gets hot, the cooler may not be dissipating heat effectively.

It is also worth checking whether the temperature reading is genuine. Use a hardware-monitoring tool such as HWiNFO and compare the CPU package, core, SoC, and individual sensor readings. If every CPU sensor reports an abnormally high temperature, the cooling or mounting is the problem. If only one sensor is far out of line, the sensor or BIOS reading could be faulty, although an automatic shutdown suggests the system is detecting a real thermal issue.

Since this appears to be a small AIO cooler, the pump could be failing or the cold plate could be clogged. Hearing the pump does not guarantee that coolant is circulating. Feel both tubes: one should generally be warmer than the other after the system has been running. If the temperatures remain extreme, replacing the AIO with an inexpensive air cooler is probably the simplest test and may be more reliable.

A working cooler from another computer producing the same result makes the mounting and contact details especially important. Remove it once more, inspect the CPU and cooler base for uneven contact, apply a small pea-sized amount of fresh paste, and tighten the mounting screws gradually in an alternating pattern. If a known-good air cooler is installed correctly and the CPU still immediately reaches 90–95°C, the CPU or motherboard temperature reporting may need further investigation.