Clearing the CMOS is a troubleshooting technique that can resolve problems with a computer failing to boot properly. This simple process resets the BIOS settings to their factory defaults. Here is a step-by-step guide on how to clear the CMOS on a desktop computer.
When to Clear the CMOS
Clearing the CMOS should be performed when:
- The computer powers on but does not boot fully into the operating system.
- The computer powers on but displays a “CMOS checksum error” or similar BIOS error message.
- A new CPU, motherboard, or RAM has been installed and the computer fails to POST.
- BIOS settings have been changed incorrectly, preventing the system from booting.
Resetting the BIOS to factory defaults can resolve boot issues caused by incorrect or corrupted settings.
Preparing to Clear the CMOS
Follow these steps before clearing the CMOS:
- Turn off all power to the computer and peripherals and unplug the power cable from the wall outlet. Wait at least 60 seconds for residual power to dissipate.
- Open the computer case and locate the CMOS battery and jumper. Refer to the motherboard manual for their exact locations.
- Ensure you are grounded by periodically touching an unpainted metal surface on the case. This prevents static discharge from damaging components.
Clearing the CMOS
There are two main methods to clear the CMOS:
Method 1: Remove the CMOS Battery
- Locate the CMOS battery on the motherboard. This is a flat, silver battery about the size of a nickel.
- Gently remove the battery from its holder.
- Wait 60 seconds to ensure all residual power is drained.
- Reinsert the battery into the holder and ensure it is securely in place.
Method 2: Use the CMOS Jumper
- Locate the CMOS jumper on the motherboard, close to the battery.
- Remove the jumper cap and place it on just one pin to short the jumper.
- Wait 60 seconds with the jumper shorted.
- Remove the jumper cap and place it back on both jumper pins.
Both methods reset CMOS by cutting power to the BIOS chip, erasing any customized settings.
Power Up and Load BIOS Defaults
After clearing the CMOS:
- Close the computer case and reconnect the power cable.
- Turn on the computer and immediately enter the BIOS setup menu.
- Load the BIOS default settings and save changes. The computer should now boot normally.
If boot problems persist even after resetting the BIOS, the issue may lie with faulty hardware rather than incorrect settings.
Troubleshooting Tips
- Confirm the CMOS battery voltage is at least 3V. Replace if needed.
- Some motherboards have a CMOS reset button instead of a jumper. Press this button to clear CMOS.
- Refer to the motherboard manual for the exact CMOS clearing procedure for your model.
- Clearing the CMOS will not delete data stored on the hard drive. Only BIOS settings are affected.
Resetting the BIOS by clearing the CMOS is a quick and simple fix for many boot up issues. Pay close attention to motherboard instructions and this process can get a computer up and running again.
