A checksum error means stored data no longer matches the check value saved alongside it. At startup that data is the BIOS settings in CMOS, and the cause is nearly always a dead coin cell. Load the defaults, then replace the battery.
- CMOS holds the BIOS settings and a checksum of them
- The BIOS recomputes and compares at every startup
- A dead CR2032 loses the settings, so the values cannot match
- A clock that resets every morning is the same fault, earlier
- A warm restart never removes power from CMOS, so it proves nothing
On this page
What it isWhat a checksum is, in one paragraph
A checksum is a small value computed from a larger block of data. Write the data, compute the value, store both. Read the data later, compute the value again, and compare: matching values mean the data is very probably unchanged, and different values mean something moved.
That is all it is, and the simplicity is why it appears everywhere. It does not repair anything, it does not say what changed, and it does not say why. It answers one question, and the answer is yes or no.
The word covers a range of algorithms of very different strength. A simple sum of bytes catches accidental corruption and nothing deliberate. A CRC catches burst errors reliably and is still not cryptographic.
A cryptographic hash is designed to survive an attacker deliberately trying to produce a match. BIOS settings use the weak end, because the threat to a CMOS chip is a dying battery rather than an adversary.
The CMOS errorThe CMOS checksum error, which is the one you have
The BIOS stores its settings in a small block of memory called CMOS. Boot order, the system clock, hardware configuration, the enabled and disabled devices. CMOS needs power to retain anything, and the computer is off most of the time, so a coin cell keeps it alive.
Alongside those settings the BIOS stores a checksum of them. At every startup it recomputes the value and compares. If the CMOS settings have been corrupted, or lost entirely because the memory went unpowered, the values disagree and the BIOS stops with a checksum error rather than starting the computer on settings it cannot trust.
The CMOS battery is the cause in the overwhelming majority of cases. A CR2032 lasts years and eventually stops holding a charge, at which point the BIOS settings are lost every time the computer is unplugged or fully powered down.
A clock that resets is the same fault. If the computer starts up with a date in 2010, or the time is wrong every morning, the CMOS battery is going even if no checksum error has appeared yet. That problem usually arrives first.
Other causes exist and are much rarer. A failed or interrupted BIOS update, a power surge, overclocking settings the BIOS could not apply, or genuine damage to the motherboard. Each of those produces the same error message, which is why the fix below starts with the cheap thing.
Clearing itClearing it, in order
1. Restart once and read the message. Some BIOS versions clear a transient error on their own at the next startup. If the error comes back, continue.
2. Enter the BIOS setup and load the defaults. The option is called Load Optimized Defaults, Load Setup Defaults or Reset to Default depending on the vendor. Loading it writes a known good set of settings to CMOS along with a matching checksum.
3. Set the date and time. They will be wrong after a reset. The system needs them correct before saving.
4. Check the boot order. The defaults do not always match how the computer is actually configured, particularly on a system with more than one drive.
5. Save and exit, then power off fully. Not a restart. Turn the computer off at the switch, and unplug it for a minute.
6. Start the computer again. If the error is gone and the clock is right, the CMOS settings held without mains power and the battery is fine. If the error is back, or the clock has reset, the battery is not holding a charge and step seven is the actual fix.
7. Replace the CMOS battery. Power off, unplug, open the case, note which way up the cell sits, and replace it with the same type. It is almost always a CR2032. On a laptop this can mean removing the keyboard or the motherboard, which is the one case where the repair is not five minutes.
8. Repeat steps two to four. Replacing the battery clears CMOS again, so enter the BIOS, load the defaults and set the clock once more.
If the error persists with a known good new battery and the defaults loaded, the problem is the BIOS itself or the board, and a BIOS update is the next repair to try before treating the motherboard as failed.
The other twoThe other two checksum errors in IT
The same word, the same mechanism, entirely different fixes. Knowing which one you have is most of the diagnosis.
| Where | What is being checked | What a mismatch means | The fix |
|---|---|---|---|
| At startup | The BIOS settings in CMOS | Settings lost or corrupted | Load defaults, replace the battery |
| On a downloaded file | The file's contents | A corrupted or tampered download | Download again, verify the source |
| On a network interface | Every received frame | Damage on the cable or the link | Cable, connector, optic, or duplex |
A file checksum mismatch means the bytes you have are not the bytes the publisher had. Usually the download was interrupted or corrupted, occasionally the file was tampered with, and the check is only meaningful if the published value came from a channel an attacker could not also alter.
A network checksum error shows up as a CRC or FCS counter on an interface, not as an error message on a screen. It means frames are being damaged in transit, and it is a physical problem in almost every case. That has its own page under CRC errors, because the fix is completely different from anything here.
PitfallsWhere people go wrong
Replacing the motherboard. The error message reads as a hardware failure and the problem is a battery in the overwhelming majority of cases. Spend the price of a coin cell before anything else.
Replacing the battery without loading defaults. Removing the cell resets CMOS, so the computer comes back with no configuration and often the same error. Enter the BIOS and load the defaults after fitting the new cell.
Ignoring a clock that resets. That is the same failing battery, earlier. Replacing it then avoids the boot failure entirely.
Testing with a restart. A warm restart never removes mains power, so CMOS stays alive and the problem is hidden. Power the computer off fully and unplug it to test properly.
Fitting a battery from a drawer. Coin cells self-discharge on the shelf. A cell that has been sitting for four years may be as dead as the one you removed.
Assuming a checksum error means data loss. It means a mismatch was detected. The BIOS settings are recreatable in two minutes, and nothing on the disk or in Windows is affected at all.
ComparisonThree strengths of the same idea, and the row they share
| Criterion | Simple checksum | CRC | Cryptographic hash |
|---|---|---|---|
| Catches accidental corruption | Yes | Yes | Yes |
| Catches a deliberate change | No | No | Yes |
| Cost to compute | Very low | Low | Higher |
| Used for firmware settings | Yes | Sometimes | No |
| Used on Ethernet frames | No | Yes | No |
| Used to verify a download | Weakly | Weakly | Yes, SHA-256 |
| Tells you what changed | No | No | No |
The last row is the one worth carrying. None of them repairs anything or identifies what moved. They answer one question, and every difference between the columns is about how hard the answer is to fool.
FAQFrequently asked questions
What is a checksum error?
A mismatch between stored data and the check value saved with it. Something changed since the value was computed, and the system refuses to trust the data.
What causes a CMOS checksum error?
Almost always a dead CMOS battery, which lets the BIOS settings disappear when the computer loses power. A failed BIOS update, a power surge and a board fault are much rarer causes.
How do I fix a CMOS checksum error?
Enter the BIOS setup, load the defaults, set the date and time, save, then power the computer off fully rather than restarting. If the error returns at the next startup, replace the CMOS battery and load the defaults again.
Which battery does a motherboard use?
A CR2032 coin cell in almost every case. Check the one you remove, and fit the same type.
How long does a CMOS battery last?
Roughly three to five years as a rule of thumb, and often far longer. A computer that stays plugged in draws far less from the cell than a system unplugged every night.
Is a CMOS checksum error serious?
No. Nothing on the disk is affected and the BIOS settings take two minutes to restore. The problem becomes serious only when it is ignored and the computer stops starting up at all.
Why does my clock keep resetting?
The same failing battery, before the checksum message appears. Replacing the cell fixes both symptoms.
Can a BIOS update cause it?
Yes. An interrupted or incorrect BIOS update leaves CMOS settings that do not match their stored value. Reflashing the correct BIOS is the fix in that case.
Why does a restart not clear it?
Because a warm restart never removes power from CMOS. Only a full power off, ideally unplugged, tests whether the BIOS settings survive without the computer running.
Is a checksum error the same as a CRC error?
The mechanism is the same and the context is not. A CRC error on a network interface means frames are being damaged on the cable, and the investigation is physical rather than firmware.
What if a downloaded file fails its checksum?
Download it again, from the publisher rather than a mirror. If it fails a second time, treat the file as untrustworthy, and remember the check only proves something if the published value came from somewhere the file did not.
Is a checksum the same as a hash?
A cryptographic hash is a checksum with much stronger guarantees. Every hash is a checksum and most checksums are far too weak to be called a hash.
Can I ignore the error and keep using the computer?
It will usually start after you press the key it asks for, with default settings and a wrong clock. In Windows that breaks certificate validation, scheduled tasks and domain logon, so it is not a state to leave a system in.
Keep readingRelated concepts
Read next · Diagnostics CRC Errors, and the Four Moves That Find the Cause The same mechanism on a network interface, where a mismatch means a cable rather than a battery. Open this next11 min- Firmware and boot · 12 min What Is a BIOS? The firmware doing the comparison, and where its settings live before any of this goes wrong.
- Cryptography · 11 min Hash Functions, and Why the Right One Depends on the Job The strong end of the same idea, and the reason a firmware checksum is not expected to stop anybody.
- Identification · 9 min How to Check Which Motherboard You Have, and Why WMIC No Longer Works When the board is talking to you at boot.