How to Fix a CMOS Checksum Error

While they might seem bad, these errors often have a quick fix

A CMOS Checksum error is a conflict between the CMOS (Complementary Metal Oxide Semiconductor) and BIOS (Basic Input Output System) that happens when you boot up a computer. It occurs when the computer isn't able to read startup information or the data does not match up.

In this guide, we explain what causes a CMOS Checksum error and offer instructions for troubleshooting and fixing the problem.

Motherboard with CMOS battery

Causes of CMOS Checksum Errors

There are several potential reasons for a CMOS checksum error, but almost all come back to the information on the CMOS being corrupt for one reason or another.

Before an operating system boots, the computer's motherboard handles many lower-level tasks, preparing the system components to run and eventually handing those tasks off to the operating system. The software on the motherboard is called the BIOS. In addition to booting up a computer, the BIOS contains several settings for its hardware, like speeds, voltages, system time, and boot priorities. The BIOS settings aren't saved on the hard drive. They're on a chip called the CMOS.

Whenever you make changes to the BIOS settings, start your computer up, or shut it down, those events are written to the CMOS. It keeps track of the data to make sure that things run normally the next time you start the computer. The CMOS stays on while the rest of the computer is off because it's powered independently by a watch battery. When the computer starts, it reads the state it was last in from the CMOS. Usually, it can read the information and restore itself without an issue. A CMOS Checksum error occurs when the computer isn't able to read that information.

One of the more common causes of a checksum error is also the simplest to solve. The battery that powers the CMOS is a watch battery, and it can run out of power. When the battery is dead, the CMOS can't store information anymore.

Power surges and sudden losses of power are other causes. If a computer doesn't have a chance to write information to the CMOS before it's abruptly powered off, it has a hard time picking up where it left off. Power surges can also cause corruption or hardware damage.

The final cause is less common, but it can happen. If the BIOS is damaged or corrupted, it will cause a mismatch between the BIOS and CMOS. It's uncommon but possible for a virus to infect and corrupt the BIOS. Still, it's more common that a BIOS update failed or the operating system updated something which caused it to get out of sync with the BIOS.

How to Fix CMOS Checksum Errors

While it is not always possible to fix a CMOS checksum error, especially in the case of hardware damage, the fix is usually simple. Follow these steps, in order, to resolve the error.

  1. Restart the computer. A normal restart usually creates a new checksum and eliminates the error. An error lingering after a normal restart requires some more work.

  2. Download and flash a BIOS update. Download the update from the motherboard manufacturer's website. Many motherboards can download an update from within the BIOS while they're plugged into your network using an Ethernet cable.

  3. Reset the BIOS. Some motherboards have a switch either on the board or on the back of the computer to reset the BIOS settings. If there isn't a switch like that, remove the CMOS battery from your system for a minute or two. The loss of power causes everything in the CMOS to reset.

  4. Replace the CMOS battery. If the cause is a dead battery, all you need is a new one. The CMOS battery is located on the computer's motherboard. On desktops, it's easy to get to, and it's only held in place with a metal clip. On laptops, you'll need to open the machine up to get to the motherboard, and that might be better left to a professional.

  5. Consult a technician or computer repair expert. If all of the above fail, the problem may be due to hardware damage. Before you buy a new motherboard or recycle the machine, have a professional check it out to be sure.