CMOS Checksum Error: What It Is and How to Fix It

It's usually just a quick fix

Motherboard with CMOS battery

Before your operating system boots, the computer’s motherboard handles a bunch of lower-level tasks, preparing all the system components to run and ultimately handing things off to the operating system. The software on the motherboard is called the BIOS (Basic Input Output System). In addition to booting up your computer, the BIOS contains a lot of settings for its hardware, like speeds, voltages, system time, and boot priorities. The BIOS settings aren’t saved on your hard drive. They’re on their own chip called the CMOS (Complementary Metal Oxide Semiconductor).

Whenever you make changes to your BIOS settings, start your computer up, or shut it down, those events are written to the CMOS. It keeps track of those things to make sure that things are running normally the next time you start your 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 tries to read 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 or it doesn’t quite match up.

What Causes It?

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

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

Power surges and sudden losses of power are another prime suspects. If your computer doesn’t have a chance to write information to CMOS before it’s abruptly powered off, it’s going to have a hard time picking up where it left off. Power surges can also cause corruption or even actual hardware damage.

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

How Can You Fix It?

It isn’t always possible to fix a CMOS checksum error, especially in the case of hardware damage, but more often than not, the fix is simple.

Before you panic, restart the computer. A normal restart will usually create a new checksum and eliminate the error. An error lingering after a normal restart will require some more work.

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

In most other cases, you can simply reset the BIOS. Some motherboards have a switch either on the board itself or on the back of the computer to reset the BIOS settings. If there isn’t a switch like that, you can remove the CMOS battery from your system for a minute or two. The loss of power will cause everything in the CMOS to reset.

If you think that it was an update, either of the BIOS or the operating system, that caused the error, you can download and flash a BIOS update from your motherboard manufacturer’s website. It’s not difficult to do, and many motherboards can download an update from within the BIOS while they’re plugged into your network via an Ethernet cable.

Hopefully, one of the solutions above solved the issue. If everything has failed so far, it may be hardware damage. Before you buy a new motherboard or recycle the machine, have a professional check it out to be sure.