What clearing CMOS actually does

Clearing CMOS resets your computer's BIOS settings back to factory defaults. The CMOS is a small battery-powered chip on your motherboard that stores BIOS configuration — things like boot order, CPU speed, RAM timing, and security settings. When you clear it, you erase those stored settings, and your computer restarts with the manufacturer's original configuration.

Most people clear CMOS to fix a computer that won't start, recover from a bad BIOS update, or bypass a forgotten BIOS password. It does not erase your hard drive, your operating system, or any files. It only resets how your hardware is configured at startup.

You should know upfront: if your computer is working normally, you do not need to clear CMOS. Clearing it will reset any custom settings you have made — like overclocking, custom fan curves, or boot device order — so only do this when you have a specific reason.

Key Takeaways

  • Clearing CMOS resets BIOS settings to factory defaults but does not touch your files, operating system, or hard drive.
  • The most common method is removing the CMOS battery from the motherboard for 10 to 30 seconds, then reinstalling it.
  • Some newer motherboards have a CMOS reset button or jumper on the board itself, which is faster than removing the battery.
  • After clearing CMOS, you will need to reconfigure any custom settings like boot order or security features.
  • If your computer still will not start after clearing CMOS, the problem is likely hardware failure, not BIOS configuration.

Removing the CMOS battery (the most common method)

This is the standard way to clear CMOS on most desktop computers. First, shut down your computer completely and unplug the power cable from the back of the case. Wait at least 30 seconds — this drains any remaining power from the motherboard. Then open your computer case. On most desktops, you remove a side panel with two or three screws or thumb latches.

Locate the CMOS battery on the motherboard. It looks like a large coin — usually silver, about the size of a watch battery. It sits in a small circular socket. Gently push the metal clip on one side of the socket outward, and the battery will pop out. Leave it out for 10 to 30 seconds. This gives the CMOS chip time to fully discharge. Then press the battery back into the socket until the clip snaps closed.

Close your case, plug in the power cable, and turn on your computer. Your BIOS is now reset. You may see a message on startup saying "CMOS checksum error" or asking you to press a key to enter setup — this is normal. Press the key to enter BIOS (usually Delete, F2, or F12 depending on your motherboard brand), check that your hard drive is recognized, and exit. Your computer will boot normally.

Using the CMOS reset button or jumper

Many newer motherboards have a dedicated CMOS reset button or jumper on the board itself, which is faster than removing the battery. Look for a button labeled "CLR CMOS", "CLEAR CMOS", or "PSWD" (password reset). If you find one, shut down and unplug your computer, then press and hold the button for 10 seconds. Plug back in and restart — your CMOS is cleared.

If your motherboard has a jumper instead of a button, you will see three small pins close together labeled something like "PSWD" or "CLR CMOS". A small plastic cap sits on two of these pins. Shut down and unplug, then move the cap to the other two pins for 10 seconds, then move it back. This achieves the same result as the button.

Check your motherboard manual to find the exact location and label. The manual is often available as a PDF on the motherboard manufacturer's website if you no longer have the physical copy. Searching "[your motherboard model] manual" will usually find it quickly.

What to do after clearing CMOS

After your computer restarts, it will boot with default BIOS settings. Your hard drive and operating system are untouched, so Windows or Linux will load normally. However, any custom settings you made in BIOS are gone. If you had changed boot order, disabled certain hardware, set a custom fan curve, or overclocked your CPU, those changes are reverted.

If you cleared CMOS to fix a startup problem, your computer should now boot. If it still will not start, the issue is likely a failed hard drive, bad RAM, or other hardware failure — not a BIOS problem. If you cleared CMOS because you forgot your BIOS password, the password is now gone and you can enter BIOS freely.

If you had custom settings you want back, you will need to re-enter them manually. Write down any settings you remember before you clear CMOS if you think you might want to restore them later. Some people take a photo of their BIOS screens before clearing, so they have a reference.

When clearing CMOS will not help

Clearing CMOS fixes BIOS-level problems: a computer that will not POST (Power-On Self-Test), a forgotten BIOS password, or corruption from a failed BIOS update. It does not fix problems that happen after your operating system loads — like Windows crashes, driver issues, or malware. If your computer starts but your operating system will not load, clearing CMOS will not solve it.

It also will not help if your hard drive is failing, your RAM is bad, or your power supply is dying. These are hardware failures that show up during POST or shortly after, but clearing CMOS does not repair hardware. If your computer beeps repeatedly on startup or shows error codes, consult your motherboard manual to decode what the beeps mean — that will tell you whether CMOS clearing is the right step.

Clearing CMOS on laptops

Most laptops do not have an accessible CMOS battery or reset button. The CMOS battery is soldered to the motherboard or sealed inside the case in a way that requires disassembly beyond what most people can safely do. If your laptop has a forgotten BIOS password or BIOS corruption, contact the manufacturer's support or a repair shop — they have tools and procedures for this that are specific to your model.

Some newer laptops have a BIOS reset option in the BIOS menu itself. If you can still enter BIOS (usually by pressing F2, F10, or Del during startup), look for an option like "Reset to Defaults" or "Load Defaults". This achieves the same result as clearing CMOS without opening the case.

Frequently Asked Questions

Will clearing CMOS delete my files or operating system?

No. CMOS only stores BIOS settings — how your hardware is configured at startup. Your hard drive, files, and operating system are completely separate and untouched. After clearing CMOS, your computer will boot normally and all your data will be there.

How long should I leave the CMOS battery out?

10 to 30 seconds is standard. Some people leave it out for a full minute to be safe, but 10 seconds is usually enough. The goal is to let the CMOS chip fully discharge so it forgets its stored settings.

My computer still won't start after clearing CMOS. What now?

If clearing CMOS did not fix the problem, the issue is likely hardware failure — a bad hard drive, failed RAM, or power supply issue — rather than a BIOS problem. Try reseating your RAM (removing it and pushing it firmly back in) and checking that all power cables are fully connected. If the computer still will not start, you may need hardware repair.

Do I need to clear CMOS if I'm replacing my motherboard?

No. Clearing CMOS is for resetting settings on a motherboard you are keeping. When you install a new motherboard, it comes with factory defaults already set, so there is nothing to clear.

Can I clear CMOS while the computer is plugged in?

You should not. Always unplug the power cable first. Even with the computer off, the motherboard still has power flowing through it. Unplugging ensures the CMOS chip is fully discharged before you remove the battery, which makes the reset more reliable.