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I have an 8th generation Core i5 processor, and it always worked great until these last two days where it doesn't go over 0.4 GHz. I've updated the chipset drivers, removed Intel thermal management, installed it again, ran Intel CPU testing (everything passed), ran stress tests with other tools, tried ThrottleStop, changed advanced power options, created my own profiles, checked the battery (it's working as usual), and tried it plugged in.

I'm kind of desperate for options at this point. Even the fans are not spinning while the CPU temperature doesn't go over 60 degrees Celsius even on stress tests. Any suggestions?

Screenshot from OpenHardwareMonitor:

I suggest you check your CPU fan: maybe it died and the chipset is smart enough to detect it and throttle down the CPU, but not smart enough to tell you what the problem is.

If you have recently changed power adapter, it might not be providing enough power for the CPU to ramp up, which could cause the throttling you're experiencing.

That was at least the case for me last time I experienced these symptoms.

Try updating your BIOS / firmware. I had a similar problem with an Acer Spin 15: The CPU ran at 0.8 GHz, which was the slowest speed it could run at.

The issue coincided with one of the big Windows 10 updates which made changes to power management. A firmware update was required to allow Windows to scale the CPU speed.

Some laptops have a quiet mode feature. It tries to reduce how much heat your computer produces and disables fans. On some laptopts it's controlled using the keyboard. It's easy to accidentally enable if it's one of the function keys alongside volume control, screen brightness, keyboard backlight, etc.

If it's not controlled by the keyboard then it's possible that it's a BIOS setting.

Your CPU is capable of running at variable clock speeds. The frequency is usually some multiple of 400 MHz or 800 MHz on consumer Intel machines. This mode locks the clock speed to its minimum value. It is supposed to make the computer draw less current and thus run cooler. (Of course with the fans disabled it won't stay very cool.)

I'm not sure if the setting can be disabled by software. From my experience with this issue on Windows 7/8 the only way I could disable it was using the keyboard.

You should also restore other power settings to their default values after toggling "quiet" or "fan-less" mode. There are power settings in Windows that will similarly throttle CPU speed to reduce power consumption. (Such as the "system cooling policy", which can be set to either active cooling (fans) or passive cooling (throttling). I think in active cooling mode both fans and throttling are used but throttling is only enabled if your computer runs hot even with fans active.)

You can see similar throttling (normally temporary) if your fans are broken or don't work well. Assuming the clock speed is stuck at one frequency (even immediately after booting when the computer is still cold) and that the CPU temperature sensors seem to work, then it's probably because quiet mode is enabled.

I experienced this a few months ago on an older Dell laptop. Tried a new power adapter, updated BIOS, drivers, Windows, etc.

Try taking out the battery and boot it with just the power adapter. This is assuming it is a laptop since it's a U series CPU. A bad battery will also cause the ridiculous throttling. The battery can be bad even if the machine doesn't report it as such.