Clearing the CMOS erases all custom settings stored in your motherboard’s configuration memory and resets them to factory defaults. This includes your clock speeds, boot order, fan profiles, date and time, and any BIOS-level passwords. Your files, operating system, and everything on your hard drives or SSDs remain completely untouched.
The CMOS (complementary metal-oxide semiconductor) is a small chip on your motherboard that holds your system’s hardware configuration. A coin-cell battery keeps this memory powered even when your PC is off. Clearing it is one of the most reliable first steps when your computer won’t boot properly or behaves strangely after a settings change.
What Gets Reset
When you clear the CMOS, every setting you or your motherboard have configured in the BIOS reverts to its original default. That covers a wide range:
- CPU and memory settings: Any overclocks, voltage tweaks, or undervolts are wiped. Clock speeds and memory timings return to stock values.
- Boot order: The system forgets which drive you told it to boot from first and falls back to the default sequence.
- Date and time: The clock resets, often to a date like January 1, 2000. You’ll typically see a prompt asking you to set this again on first boot.
- Fan curves and thermal profiles: Custom fan speed settings revert to the motherboard’s defaults.
- BIOS passwords: Any supervisor or user passwords set within the BIOS are removed entirely.
- Storage mode: The SATA configuration (AHCI, RAID, etc.) returns to its default, which can occasionally prevent your OS from booting until you change it back.
Nothing on your drives is affected. The CMOS only stores low-level hardware configuration, not personal files, applications, or your operating system. Think of it as resetting a thermostat to factory settings: the temperature in the house doesn’t change, just the instructions the thermostat is following.
Problems a CMOS Reset Can Fix
A failed overclock is the most common reason people clear their CMOS. If you pushed your CPU clock speed or memory timings too far, your system may crash, freeze, or refuse to turn on at all. A bad undervolt can even prevent the machine from completing its initial power-on self-test. Clearing the CMOS undoes those changes and lets you start fresh.
A black screen on startup is another classic fix. When faulty BIOS settings prevent your graphics card or display output from working, a reset eliminates the bad configuration and restores default video output. This also applies to hardware detection problems: if you’ve swapped components and the system can’t figure out what’s connected, reverting to defaults forces it to re-detect everything.
BIOS corruption, though less common, can cause erratic behavior or boot loops. A CMOS clear wipes the corrupted data and gives the system a clean slate. And if you’ve forgotten a BIOS password that locks you out of the setup screen, clearing the CMOS removes it without needing any special software.
Persistent wrong date and time after every restart can also point to a CMOS issue. A reset clears outdated settings and lets the system re-sync on boot.
How to Clear the CMOS
There are two common methods: using a jumper on the motherboard or removing the battery. Both require you to power off the computer, unplug the power cable, and open the case. Ground yourself by touching the metal case frame before reaching inside to avoid static damage.
Jumper Method
Most motherboards have a three-pin CMOS jumper located near the coin-cell battery. The jumper cap normally sits on pins 1 and 2. To clear the CMOS, move it to pins 2 and 3, wait one to five minutes, then move it back. Your motherboard manual will show the exact location, which varies by model. Some newer boards have a dedicated “Clear CMOS” button on the rear I/O panel, which does the same thing without opening the case.
Battery Removal Method
If you can’t find the jumper or your board doesn’t have one, you can pop out the CR2032 coin-cell battery on the motherboard. With the power cable unplugged, remove the battery and wait at least a few minutes. Some guides suggest 10 seconds is enough, but erring on the longer side ensures the residual charge in the board’s capacitors fully drains and the settings actually clear. Snap the battery back in when you’re done.
What to Do After a Reset
When you power on after clearing the CMOS, the system will likely pause with a message about the date and time being incorrect or settings being lost. Press the key indicated on screen (usually F2 or Delete) to enter the BIOS setup. From there, you’ll want to reconfigure a few things.
Set the correct date and time first. Then check your boot order to make sure the system boots from the right drive. If you were running your storage in RAID or a specific SATA mode, verify that setting matches what your operating system expects, otherwise you may get a blue screen or boot failure. If you had XMP or DOCP enabled for your RAM to run at its rated speed, you’ll need to turn that back on as well. Any CPU overclocks or fan profiles will need to be reconfigured from scratch.
It’s a good idea to note your preferred BIOS settings before you clear the CMOS, or take photos of each settings page with your phone. Some motherboards let you save BIOS profiles to a USB drive, which makes restoring everything much faster.
When Your CMOS Battery Is Dying
A CMOS battery typically lasts 2 to 10 years. When it starts to fail, you’ll notice symptoms that look a lot like what happens after a CMOS reset, except they happen on their own, over and over. The most obvious sign is your clock resetting to a default date every time you shut down. Custom BIOS settings may also revert to defaults after each restart.
Other symptoms include error messages mentioning “CMOS” or “BIOS checksum” during startup, slower boot times as the system reconfigures settings from scratch each time, and intermittent hardware detection failures. In some cases, a dead battery can cause boot loops or prevent the system from completing its power-on self-test entirely. Replacing the battery is simple and inexpensive: it’s a standard CR2032 coin cell available at any electronics or grocery store.
CMOS on Modern Systems
The term “CMOS” dates back to older BIOS-based systems, but the concept carries forward into modern UEFI firmware. Today’s motherboards store their settings in non-volatile memory (NVRAM) rather than a battery-dependent CMOS chip, but the reset process works the same way. The jumper and battery removal methods still clear the stored configuration on virtually all consumer motherboards. Some manufacturers, like Dell, label the relevant jumpers as “RTCRST” or “NVRAM” instead of “CMOS,” but the outcome is identical: your firmware settings return to defaults, and your drives stay untouched.