Cause of CMOS Checksum Error

By Editorial Team

Updated December 13, 2019

The CMOS (Complementary Metal Oxide Semiconductor) holds a computer's BIOS settings and the system time. The operating system stores a number in the CMOS upon shutdown that will be checked against a number generated by the BIOS on startup. If these numbers do not match, the user receives a checksum error. A CMOS checksum error is usually not a serious problem, and these errors can often be fixed by replacing the CMOS battery.

What Checksum Errors Mean

When a user starts his computer, the computer will load its BIOS settings from the CMOS. If a computer cannot read these settings correctly, it generates a checksum error and it issues a number of beeps that vary according to the BIOS manufacturer's error codes.

Battery Losing Power

CMOS batteries last for several years, but the computer may not be able to load the saved settings correctly if the CMOS battery needs to be replaced. The computer will load the factory default BIOS settings if the user-saved settings can not be used.

Corrupted Firmware

Occasionally, corrupt firmware can cause a checksum error. The process of flashing the BIOS usually fixes this. Flashing a BIOS is technical jargon that simply means you're loading a new version of the BIOS (Basic Input/Output System) onto the system's firmware.


Some viruses cause the checksum number generated by the computer on boot-up not to match the number stored by the CMOS. Removing the virus that caused the checksum value to change fixes the problem.

Improper Shutdowns

Sometimes, if the computer does not power down properly, the saved number might not match the one generated during a system restart. Shutting down the system properly during its next boot should fix this problem.

Getting a New Battery

PC repair shops can order a new CMOS battery for most models of computers. These small, circular batteries are not difficult to replace and usually just sit in a slot on top of the motherboard.