Considered the first step for many PC issues after confirming the system is plugged in, clearing the CMOS is a valuable step in the troubleshooting process.  This guide will cover several common ways to reset the CMOS on your PC.

What is CMOS?

Short for Complementary Metal-Oxide-Semiconductor, CMOS refers specifically to a tiny, battery-powered volatile memory chip (often paired with a Rea-Time Clock or RTC which stores foundational system settings necessary for the system to start.  Because it is volatile memory, if the CMOS is ever without power for an extended period of time it will be erased to a clean slate.

Preparing to clear CMOS

To prepare your PC to clear the CMOS, first start by shutting down the PC and making sure to drain any residual power by doing the following:

• With the PC powered off, unplug the AC power cable connecting the Power Supply to your main power (wall outlet or surge protector).
• Press and hold the Power On button for ~30 seconds.

How do I clear CMOS?

With the power drained, there are three common ways to clear the CMOS which will be listed according to difficulty.  It is worth noting that the more difficult methods tend to be able to more reliably clear the CMOS without issue.

Clear CMOS Button

Depending on your motherboard model, your system may have a dedicated Clear CMOS button with a label such as CLR_CMOS as shown on the NZXT motherboard above.  To clear the CMOS, press and hold this button for ~30 seconds while the PC has no power.  After releasing the button, reconnect the PC to main power and try starting the system.

CMOS Jumper Pins

One of the oldest and most common ways to clear the CMOS is to trip a pair of jumper pins located on the motherboard.  These pins go by a number of different names which may include:

• RESET_BIOS
• CLR_CMOS
• CLEAR_CMOS
• CLR_BIOS
• JBAT1

Once you have located the jumper pins according to your motherboard user's manual, use a conductive object to create a bridge or jumper between the two pins.  Some motherboards may have a 3-pin header with an included jumper, but for many users the tip of a metal screwdriver or other conductive object such as a house key can be used to bridge the pin. 

As shown above, touch both pins with your conductive object for ~30 seconds to bridge the pins.  After releasing the bridge, reconnect the PC to main power and try starting the system.

Removing the CMOS Battery

The most effective way to reset the CMOS is to temporarily remove the CMOS Battery from the system.  This is most commonly a CR2032 button cell battery located somewhere on the mainboard as shown above.  To find the exact location of the battery, please refer to your motherboard user's manual.

Most CMOS batteries willb e held in using a simple spring-latch that hold the battery in place.  To release the battery, compress the spring latch from the inside of the socket into the squared cut-out as shown above.  Once compressed, the battery should lift on one side for easy removal or will pop out entirely.  If you are having difficult removing the battery, you can use the tip of a screwdriver or pen to more easily access the spring.

If you have an ITX motherboard, the motherboard may instead use an alternative battery such as one connected via a wire.  For more information, please refer to your motherboard user's manual.

What do I do after clearing the CMOS?

Once the CMOS is cleared and you start your PC, you'll likely get a message informing you that the system settings have changed.  You will need to go into the UEFI/BIOS and make sure that your system settings are correct before you start using the PC again.  Normally enabling the Optimized Default Settings is sufficient for most users, however depending on your system you may need to configure additional setting such as Secure Boot.  For more information on these settings, please refer to your motherboard user's manual.