CMOS checksum error is caused when the CMOS values are incorrect or corrupted. This issue can occur because of: - Bad or old CMOS battery: Reboot the computer. If the error still occurs after rebooting, enter CMOS setup and check all values, this includes verifying that the time and date are correct. Once everything has been verified, make sure you save and exit the CMOS setup. If everything appears to be proper and the issue persists, try resetting the CMOS values to the defaults and/or Reset Configuration Data Note: If this issue continues to occur after you turn off your computer off it is possible that the CMOS battery may be weak or already used up.
More details about replacing CMOS values can be found on your motherboard's manual - BIOS update: If you had recently updated your BIOS of the computer, the CMOS values could have been reset. Enter the CMOS setup and re-enter all the appropriate values. More details about entering the BIOS setup can be found on your motherboard's manual. If you are unable to enter into the BIOS setup or after running a BIOS update, your computer no longer keeps its values, it is possible that an incorrect BIOS update may have been installed for your computer or the version may be corrupted.
We suggest you try to update the BIOS again or roll back to an earlier version. - Disconnecting power supply before properly shutting down the computer: If the power had been disconnected to the computer while running, it is possible that this could have corrupted the CMOS. Therefore, ensure that the computer is properly shut down before switching off the power supply. If you have a laptop, ensure that the battery is at least 25% charged before switching off the power supply.
If the CMOS has become corrupted, enter the correct values in CMOS setup and save and exit the CMOS. This would surely solve your problem.
John Cicero has written articles on topics such as CMOS Checksum Error, BIOS and Runtime Error for The Tech FAQ.