Steps to Fix CMOS Checksum Error On Dell XPS Laptop by Dell Laptop Support expert. Troubleshoot CMOS Checksum Error On Dell XPS Laptop. CMOS checksum error generally occurs when the default factory settings have been restored to DELL Laptop / Desktop startup, meaning the DELL system. A checksum is calculated as a code detection error to save the BIOS settings stored in the memory of the CMOS. Means that the CMOS contains BIOS-related data and may have been reset. The problem could also arise due to the depletion of the CMOS battery or even if the CMOS battery may not function properly or the battery life may have expired, the BIOS may have been updated by a virus, and if the computer may not have been properly shut down, the memory size of the CMOS battery may have been incorrect, and the default CMOS checksum error loaded and many others.