- CMOS technology is widely used in computer circuitry, especially in anything resembling a microprocessor.
- Of course, the technology is delicate, so the occasional error does occur, such as the CMOS checksum error in Windows 10.
- This is just one of many similar guides that we've covered in our dedicated Hub for Troubleshooting System Errors.
- Even more helpful guides can be found by browsing our dedicated Windows 10 Errors page.
The CMOS checksum error is essentially a conflict between the CMOS and the BIOS that happens when you boot your PC.
When you shut down your PC the CMOS records a number that must match another that the BIOS generates when you start up the computer again.
If these two values don’t match the PC may return a CMOS checksum error. This is an error detection mechanism is what is referred to as checksum, otherwise known as a form of redundancy check for picking errors in data.
How the CMOS and BIOS work
Where the OS manages the software running on a computer, the BIOS is more a firmware that checks the computer’s RAM, processor, and it’s peripheral hardware components so the OS can load properly.
The BIOS is where important system data such the time and date are verified with the CMOS. It runs independently of whatever OS you run on your PC and manages the relationship between all of the PC‘s hardware elements.
But, in effect, the OS cannot operate without the BIOS as it is where all the operating system‘s drivers are loaded.
The BIOS itself is stored in the CMOS (Complementary Metal Oxide Semiconductor). So, in effect, the CMOS holds all of the BIOS‘ settings.
When the BIOS fails, starting or booting the computer becomes problematic as it cannot reconcile the BIOS settings prior to the computer switching off and on startup.
Causes of the CMOS checksum error
There are different reasons why a CMOS checksum error would occur. And different computers deal differently with the error. Let’s look at a few causes of the CMOS checksum error before suggesting a couple of fixes;
Out of date or corrupted BIOS
After a while, a PC‘s BIOS will need to be updated to stay in sync with the operating system and all the hardware components you use with it.
But sometimes the cause of the error will be a lot more sinister as some viruses can discreetly change the value checksum settings and corrupt the BIOS. This will cause the CMOS checksum error to pop up.
The computer requires some form of power to enable the CMOS chip to retain all of the BIOS settings during times the PC is switched off. That power is provided by a small battery.
And, unfortunately, that battery will die at some point. The BIOS will thus not be able to return the correct checksum value and the computer will have to use the BIOS default settings, hence the error.
Bad PC shutdowns
Not shutting down the computer the right means you don’t give it enough time to generate the checksum number it will need to verify against the BIOS when the computer boots again.
This will cause the CMOS checksum error. In this circumstance however the error is usually temporary as the PC will boot correctly the next time you properly shut your machine down.
A power surge or damaged motherboard
It is also not uncommon that a power surge or even physical damage could behind the CMOS checksum error. Something like this shouldn’t be easy to miss.
The best course of action will be to take the computer to a repair technician and have the PC repaired.
How do I fix CMOS checksum errors on Windows 10?
- Restart the computer
- Replace the battery
- Fix your CMOS battery’s terminals
- Reset your BIOS
- Update the BIOS
- Replace the motherboard
Every time a user starts their computer it checks the BIOS settings in the CMOS to verify the checksum values. If these match the computer will start normally.
If they don’t the PC will return the CMOS checksum error. Depending on your brand and model, the PC will sound a beeping warning.
However, on some computers, you will get a warning but the PC will revert to the default BIOS settings and proceeds to boot normally. When it doesn’t you get the error message. But there are several solutions you can try to fix the issue.
Sometimes abruptly cutting the power to your machine will cause the CMOS checksum error the next time you boot up.
Even switching the computer off using the power button without following the normal shutdown procedure can also corrupt the BIOS and cause this error to pop up.
If you suspect this could be the reason the error has OK, simply follow the computer’s instructions and restart the computer after it’s finishing booting.
The CMOS checksum error should not appear again. Just remember to shut down the correct way next time.
2. Replace the battery
In most cases, however, this error message will continue to pop up every time you start the machine, regardless of whether you shut it down correctly the last time. The error will almost always advice to press F1 and reset the time and date.
In such cases, the CMOS battery is usually the culprit. This is a small battery on the motherboard that provides power to the CMOS so it remains powered during the times you switch off the power supply to the computer.
In some laptops, you can find this small battery in a small compartment under the PC. If the voltage in the battery is low chances are you will get the CMOS checksum error on startup.
Replacing this battery may be all it takes to fix this error. You will only have to correct the date and time one last time.
The easiest way to tell if the CMOS needs replacing is to keep your PC plugged in after you shut it down. If its a laptop you can make sure it has at least 25 percent of battery power.
If you don’t get the CMOS checksum error again the next time you boot up, then you will know for sure your CMOS battery needs replacing.
3. Fix your CMOS battery’s terminals
A common issue that people have faced is the error message continued to pop up even after they have replaced the CMOS battery.
Normally if the battery has more than 3 volts the CMOS should function without a problem and return the correct BIOS settings.
It could still be a power problem, just not the battery itself. Rather the negative and positive pins that draw power from the battery may not be connecting properly. The battery would thus be loose and not supplying any power.
Remove the battery and bend the negative pin-up and also bend the positive pin in, or whatever ensures both pins have firm contact with the battery.
Afterwards, replace the battery and check if it fits snugly with both terminals properly connected. Put everything back together and restart the machine. You should be good to go.
In the cases where you suspect your BIOS has been corrupted by malware or due to a power cut, the best option may be to reset the BIOS to its default or factory settings.
This will also work where the CMOS checksum error is a consequence of a BIOS update that went wrong. Remember to run a virus scan if you suspect an infection
To reset your BIOS start hitting the F10 key as soon as you press the PC’s power button. The procedure is essentially the same with most computers, but on HP laptops you may need to use the F2 key.
You should see options for either Setup or Boot options on the bottom of the screen. Choose setup.
Once the blue setup screen comes up, strike the F9 key to reset the BIOS. Click Yes to confirm the reset. On some PCs, you can find options like Restore Security Settings to Factory Settings under the Security tab.
Press the F10 key to save before you exit.
Now, a BIOS update may, in fact, be called for where there is a newer version available. Or there may be a new peripheral device the PC fails to recognize, like a USB keyboard.
Trying to reset the BIOS when the keyboard itself isn’t recognized by the BIOS will fail.
Updating or flashing the BIOS may mean just visiting the manufacturer’s website and checking if there is an updated one available. If available, just download and run.
Some PC will also give you the option to update the BIOS right off the BIOS Setup utility. Just select the option and complete the update from there.
Remember though, updating or flashing the BIOS means you are overwriting it with new information. Be sure of what you are doing, flashing your PC may damage your it.
It is advisable to seek the help of a knowledgeable technician and do not update the BIOS unless it is really necessary.
There have actually many instances where users updated the BIOS, but with the wrong values. This will also result in the CMOS checksum error popping up on startup.
The BIOS update may also not go as smoothly due to a power cut or other reasons, in which cases the BIOS may also get corrupted and the computer won’t boot.
To fix it you may have to reset it to its default settings, as we have discussed above.
In the end, if none of the previous troubleshooting steps worked, it all points towards a critical motherboard malfunction. We can’t say with certainty what exact problem behind the malfunction, but the electric surge is the most viable one.
You can run a few tests, but chances are you won’t be able to do much except contacting a professional or replacing the motherboard. Not the best of news, nor a solution per se, but things like this tend to happen.
The CMOS checksum error can be a real irritation if you don’t get it fixed. And that irritation will grow into real frustration if you fail to find a workable solution to the problem.
Hopefully, the solutions we have discussed here will help you to fix the error and get your PC behaving again.
FAQ: Learn more about CMOS
- What is CMOS used for?
CMO is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits, and are there prevalent in PCs.
- What is the difference between CMOS and MOS?
MOS stands for Metal Oxide Semiconductor, while CMOS means Complementary MOS.
- How long do CMOS batteries last?
It all depended on the level of use, but the average lifespan of a CMOS battery is 5 years.
Editor’s Note: This post was originally published in March 2019 and has been since revamped and updated in June 2020 for freshness, accuracy, and comprehensiveness.Editor's Note: This article was originally published in October 2017 and was revamped and updated in June 2020 for freshness, accuracy, and comprehensiveness.