Owners of Microsoft's Surface Pro 6 and Surface Book 2 systems are finding themselves stuck at Pentium 2 speeds, as numerous user complaints indicate that the ultra-portables are throttling the processor down to 400 MHz, a state that—in some instances—persists across reboots. While similar issues with Surface devices have occurred in the past, reports of issues have increased in frequency following a firmware update for the Surface Pro 6.

The throttle-lock appears to be caused by an Intel CPU flag called BD PROCHOT (bi-directional processor hot), which can be set by any peripheral, telling the processor to throttle down in order to decrease system temperature—a useful flag in cases where the CPU is operating within thermal limits, but other components tied to the CPU are running too hot, because of the demands placed on other components by processes on the CPU.

SEE: Windows 10 apps: Which should you keep and which should you dump? (free PDF) (TechRepublic)

While the BD PROCHOT flag is not unique to Surface devices—or even particularly new—the outsized number of reports of throttling issues on Microsoft Surface systems, combined with the timing of reports relative to the issuance of new firmware, implies a correlation, at minimum.

Nearly decade-old reports of BD PROCHOT resulting in throttling on gaming-focused ASUS notebooks also indicate that this functionality is a known quantity.

Determining what component is (erroneously) setting BD PROCHOT is difficult, with users of Surface Pro 6 systems noting that throttling disables after disconnecting from the Surface Dock or other AC adapter, while others point to third-party antivirus software as the culprit.

ThrottleStop is a third-party utility that (among other features) intercepts changes to MSR 0x1FC Bit 0, which controls the BD PROCHOT flag, preventing Surface devices from throttle-locking. If a Surface system genuinely is overheating, it will simply shut down to prevent damage.

"We are aware of some customers reporting a scenario with their Surface Books where CPU speeds are slowed," a Microsoft spokesperson told TechRepublic. "We are quickly working to address via a firmware update."

The throttle-locking issue is the latest issue to affect Microsoft Surface devices, which have been subject to a variety of software or firmware-related problems since their introduction. Prior to this, the Surface Book 2 encountered issues with the GPU disappearing in the Windows 10 May 2019 update, and a faulty patch causing widespread BSODs. The Surface Pro 5 experienced issues with ACPI drivers, causing random hibernation or sleep mode to invoke randomly.

Conversely, the Surface Pro 4 and Surface Book would frequently not wake up from sleep mode at all, and the Pro 4 experienced violent display flickering. Power issues on the Surface Pro 3 resulted in batteries draining during sleep and refusing to hold a charge, and the power cord on the Surface Pro, Pro 2, and Pro 3 were recalled due to risks of electrical fire.

Reliability issues with Surface devices led Consumer Reports to withdraw "Recommended" ratings in August 2017, regaining the distinction in September 2018.

For more, check out TechRepublic's cheat sheet for the Microsoft Surface Pro 6.

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This article was originally published on TechRepublic.