How to Clear CMOS on a PC | Fix No POST and Boot Problems

How to Clear CMOS on a PC

A complete Pink Parsnip Computers guide to resetting your motherboard BIOS when your PC will not boot, display an image or successfully complete POST.

Pink Parsnip Computers guide showing how to clear CMOS using the motherboard battery and clear CMOS pins
The CMOS battery and JBAT or CLR_CMOS header are normally found directly on the motherboard.

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Follow Adam from Pink Parsnip Computers as he demonstrates how to locate and bridge the clear CMOS pins on a desktop motherboard.

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Before We Begin

Every PC supplied by Pink Parsnip Computers is fully built, tested and stress tested before leaving us.

Where suitable, we also enable a memory performance profile such as XMP or EXPO. These profiles allow compatible RAM to run at its intended performance speed rather than the slower motherboard default.

XMP and EXPO are normally reliable. However, memory training, BIOS changes, overclocking or an unexpected crash can occasionally cause instability.

In some cases, the computer may turn on but fail to boot or fail to complete POST. Clearing CMOS is one of the quickest and easiest troubleshooting steps you can try.

What Does Clearing CMOS Mean?

Your motherboard stores its BIOS or UEFI configuration so that it remembers how the computer should start and how the installed hardware should operate.

Clearing CMOS resets these motherboard settings back to their factory defaults. This can remove an unstable memory profile, incorrect overclock or incompatible BIOS setting that is preventing the PC from starting correctly.

Settings It May Reset

  • XMP or EXPO memory profiles
  • CPU and memory overclocking
  • Custom fan curves
  • Boot order
  • Resizable BAR
  • Secure Boot settings
  • TPM or firmware security settings
  • Other manually configured BIOS options

What It Does Not Delete

  • Windows
  • Your games
  • Photographs and documents
  • Applications
  • Files stored on your SSD or hard drive

Clearing CMOS resets the motherboard configuration. It does not wipe your storage drive.

What is POST? POST stands for Power-On Self-Test. It is the motherboard's initial hardware check before Windows begins loading.

Important Safety Information

  • Completely switch the computer off and remove the power cable before bridging any motherboard pins.
  • Never bridge random pins. Confirm that you have located the correct clear CMOS header.
  • Use a small metal screwdriver with an insulated handle.
  • Touch an unpainted metal part of the computer case before reaching inside to help discharge static electricity.
  • Never force the CMOS battery, graphics card or any motherboard component.
  • Laptop, mini PC and proprietary computer designs may use a different procedure.

If you are unsure, stop and check the motherboard manual or contact an experienced computer technician.

Before You Start

What You Will Need

  • A small metal screwdriver
  • Access to the inside of the computer
  • Your motherboard model number
  • The motherboard manual where possible
  • A few minutes of uninterrupted time
  • Your BitLocker recovery key where applicable

Locate the Clear CMOS Header

The clear CMOS connection is normally a small two-pin header on the motherboard. It may be positioned close to the silver CMOS battery or along the motherboard's bottom edge.

Common labels include:

  • JBAT1
  • CLR_CMOS
  • CLEAR CMOS
  • CLRTC
  • CMOS_RST

The exact name and position depend on the motherboard manufacturer. Search for your motherboard model and download its manual if you cannot clearly identify the correct pins.

On many motherboards, the graphics card covers the CMOS battery. The separate clear CMOS header may still be accessible without removing the graphics card.

Diagram identifying a CMOS battery, JBAT clear CMOS pins, motherboard debug LEDs and buzzer speaker
Look for the CMOS battery, clear CMOS header, diagnostic LEDs and motherboard speaker connection.

How to Clear CMOS Using the Two-Pin Header

  1. Shut the Computer Down Shut Windows down normally where possible and wait for the computer to turn off completely.
  2. Switch the Power Supply Off If the power supply has a rear switch, move it to the off position, normally marked with a zero.
  3. Remove the Power Cable Completely disconnect the mains power cable from the back of the computer.
  4. Drain the Remaining Power Press and hold the computer's normal power button for approximately 10 seconds. The PC should remain disconnected while you do this.
  5. Locate the Correct CMOS Pins Remove the side panel and find the two-pin JBAT1, CLR_CMOS, CLRTC or equivalent header. Check the motherboard manual if you are not completely certain.
  6. Bridge Both Pins Gently touch the metal tip of the screwdriver against both pins at the same time. You do not need to press down. The screwdriver only needs to create a connection between the two pins.
  7. Hold for 5 to 10 Seconds Keep the screwdriver touching both pins for approximately 5 to 10 seconds, then remove it.
  8. Reconnect and Power On Replace the side panel where appropriate, reconnect the power cable, switch the power supply on and press the computer's power button.
The first boot may take longer than normal. The motherboard may need to retrain the memory and detect the installed hardware again. Debug lights may cycle several times and the computer may restart during this process.

Give the computer several minutes before deciding that the reset has not worked, particularly on modern DDR5 systems.

Pink Parsnip Computers five step CMOS reset guide showing shutdown, unplugging, draining power, bridging pins and powering on
Clear CMOS in five simple stages: shut down, unplug, drain power, bridge the correct pins and restart.

Other Ways to Clear CMOS

The two-pin header is one of the most common methods, but some motherboards provide other options.

Method One: Dedicated Clear CMOS Button

Premium motherboards may have a dedicated Clear CMOS button either directly on the motherboard or on the rear connection panel.

Shut the computer down and follow the exact procedure shown in the motherboard manual. Do not assume every manufacturer's button operates in the same way.

Method Two: Remove the CMOS Battery

Shut the PC down, switch off the power supply, remove the power cable and drain the remaining power.

Carefully release the retaining clip and remove the silver CR2032 battery. Wait approximately 5 to 10 minutes before reinstalling it with the positive side facing the correct direction.

Do not force the battery. On some systems, the graphics card may need to be removed before the battery can be reached safely.

Method Three: Three-Pin Jumper

Some older or specialist motherboards use a three-pin header with a removable jumper cap.

The cap normally moves temporarily from one pin position to another, but the exact procedure varies. Follow the motherboard manual and do not guess the correct position.

What to Do After the PC Boots

A successful reset means the motherboard is now using its default settings. You may see a message telling you that the BIOS has been reset or asking you to press a key to enter setup.

Restart the PC and repeatedly press Delete or F2 to enter the BIOS if required.

You may need to check or reconfigure:

  • System date and time
  • Correct Windows boot drive
  • TPM or firmware TPM
  • Secure Boot
  • XMP or EXPO
  • Resizable BAR
  • Custom fan curves
  • Virtualisation settings
Do not immediately re-enable XMP or EXPO. First allow the PC to boot using the default memory settings and confirm that it is stable. If the computer fails again immediately after the profile is enabled, leave it disabled while the memory is investigated.

BitLocker Recovery

Resetting firmware security settings can occasionally cause Windows to request a BitLocker recovery key. This does not necessarily mean that your files have been lost.

Enter the recovery key associated with the Microsoft account or organisation that encrypted the computer.

Motherboard diagnostic guide explaining CPU DRAM VGA and BOOT LEDs and motherboard buzzer beep codes
After clearing CMOS, watch the motherboard diagnostic LEDs and listen for any beep code.

Read the Motherboard Debug Lights

Many motherboards have small diagnostic LEDs, normally positioned near the top-right corner. These lights cycle as the computer checks each major component.

If the motherboard remains stuck on one light, it can help identify the next area to investigate.

CPU

Processor, CPU power, motherboard socket, BIOS compatibility or motherboard problem.

DRAM

Memory detection, memory training, incorrect seating or unstable RAM settings.

VGA

Graphics card, PCIe seating, GPU power, display cable or monitor connection problem.

BOOT

Boot drive, Windows boot installation or incorrect BIOS boot order.

If the DRAM Light Remains On

  1. Switch off and disconnect the PC again.
  2. Remove all but one memory stick.
  3. Install that stick in the motherboard's recommended single-stick slot, which is commonly A2.
  4. Try each memory stick separately.
  5. Check that each module is fully clicked into place.

If the VGA Light Remains On

  1. Check that the graphics card is fully inserted.
  2. Check every PCIe power connector.
  3. Connect the monitor cable directly to the graphics card.
  4. Check the monitor's selected input.
  5. Try another cable or display where possible.
A VGA debug light during POST is normally a hardware detection, power or display connection issue. Windows graphics drivers have not loaded at this stage.

Motherboard Speaker and Beep Codes

Some PCs have a small motherboard speaker or buzzer fitted. On many systems, a single short beep indicates that POST completed successfully.

Multiple beeps, a repeating pattern or no beep may provide additional clues. Beep codes vary between motherboard and BIOS manufacturers, so always check the manual for the exact system.

Frequently Asked Questions

Does clearing CMOS delete Windows?
No. Clearing CMOS resets motherboard BIOS settings. It does not erase Windows, games, photographs or personal files from the SSD or hard drive.
Will clearing CMOS remove XMP or EXPO?
Yes. The memory profile will normally return to its default setting. This is useful when an unstable profile is preventing the computer from completing POST.
How long should I bridge the pins?
For a standard two-pin clear CMOS header, approximately 5 to 10 seconds is normally sufficient while the power cable is completely disconnected.
Why is the first boot taking so long?
The motherboard may be detecting the hardware and retraining the memory. Modern DDR5 systems can take noticeably longer and may restart several times.
Can clearing CMOS fix a black screen?
It can help when the black screen is caused by an unstable memory profile, incorrect BIOS setting or failed memory training. It will not repair a physically faulty component.
What if the computer still will not POST?
Note which diagnostic light remains illuminated or record the beep pattern. Then test the relevant component, beginning with one memory stick, power connections, graphics card seating and display cables.
Should I remove the graphics card?
Not necessarily. If the clear CMOS header is accessible, use that instead. Only remove the graphics card if access is required and you are confident doing so safely.

Still Need Help?

Pink Parsnip Computers builds, tests, repairs and upgrades gaming PCs throughout the UK.

When contacting us, please include your motherboard model and tell us which CPU, DRAM, VGA or BOOT light remains illuminated.

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