CMOS-Reset

How to reset the CMOS?

CMOS-Reset instructions for PC-Mainboards and Notebooks

There are of course other situations, but the above are by far the most common reasons to make a CMOS reset.

What is CMOS?
On which computers can i reset a BIOS-Password?
CMOS-Reset with Jumper
CMOS-Reset without Jumper
CMOS-Reset did not solved the problem! What can i do?

I assume no liability for the accuracy, completeness or topicality of the following instructions. These instructions describe only in general, how to erase the CMOS on PC-Mainboards and Notebooks. It may be completely different for your computer!

First of all: the CMOS is not the BIOS!
The CMOS (Complementary Metal-Oxide Semiconductor) is a static Random Access Memory (SRAM), which stores the BIOS values. The CMOS will lost its data when removing the voltage/battery (CMOS-Battery, see picture). The duration for a complete loss of data could differs between some seconds, hours, or several days. So it usually won't help to remove the CMOS-Battery for a short time! For this reason, most of all PC-Mainboards have a jumper to immediately reset the CMOS. Notebooks don't have any jumper.

YES, on PC-Mainboards you can clear the BIOS-Boot password resp. BIOS-Setup password by resetting the CMOS.

NO, on Notebooks you can not clear the BIOS-Boot password resp. BIOS-Setup password by resetting the CMOS, because most of all notebooks have a better theft protection. They have an additional small, hidden IC which stores the password. That means: the password is not stored inside the CMOS! Other notebooks are able to store the password on a hidden hard disk sector. Your only chance is to contact the support of your notebook manufacturer. Depending on the manufacturer, you have to send him the serial number of the notebook and you'll get (in many cases free of charge) an individual master password to unlock the notebook. Some other manufacturers even offer only the replacement of the notebook's mainboard - which will be really expensive!

Most mainboard manufacturers have a download section on their websites where you can find the manual of your specific mainboard type. Inside the manual there should be a layout drawing which shows the jumper position.

Read the manual to find out how you can reset the CMOS! Because the procedure can be very different! For instance, most mainboards have to be completely dead, but there are others where you have to reset the CMOS when power is on.

In most cases you can find the jumper (green jumper cap, see picture) nearby the CMOS-Battery. There should be a label like JBAT1, RTCLR, CLRCMOS, CLRCMS, CMOS_CLEAR, Clear RTC, or similar.

There are 4 different CMOS-Jumper types:

3-Pin CMOS-Jumper: The connector strip has 3 pins and the default jumper position is pins 1+2 (see picture). To reset the CMOS move the jumper cap to position pins 2+3.

2-Pin CMOS-Jumper: Depending on the default position, you have to remove or place the jumper.

Soldered points: Sometimes there is no connector strip and you'll only find 2 soldered points which must be bridged (e.g. with a paper clip) to reset the CMOS.

CMOS-Reset-Button: On newer mainboards you'll find different buttons for e.g. Power, Reset and CMOS-Reset. The CMOS-Reset-Button works like a normal CMOS-Reset-Jumper.

1. Turn OFF the computer and unplug the power cord (on notebooks remove all accus).
2. Press the Power On/Off button (PC-Case) 3-5 times to discharge the mainboard condensers.
3. Remove the coin cell battery (CMOS battery).
4. Move the CMOS-Jumper to his clear-position (read your mainboard manual to find the jumper).
5. Wait ~ 15 seconds (the longer the better).
6. Move the CMOS-Jumper to his default position.
7. Insert the coin cell battery (CMOS battery).
8. Plug the power cord (on notebooks insert all accus).
9. Turn ON the computer, enter the BIOS-Setup and load the "Setup Defaults/Optimized Settings".
10. Save the BIOS settings and restart the computer.
11. No success? Repeat the steps 1-10.

Never turn on the mainboard during a CMOS-Reset!
It could cause a short circuit which would damage your mainboard!

If your mainboard has no jumper (like most of all notebooks) to reset the CMOS, you can try to remove the CMOS-Battery for a few days and hope that this will clear the CMOS. If you don't like to wait, you can try the following instructions, but as I said at your own risk.

1. Turn OFF the computer and unplug the power cord (on notebooks remove all accus).
2. Press the Power On/Off button (PC-Case) 3-5 times to discharge the mainboard condensers.
3. Remove the coin cell battery (CMOS battery).
4. Insert a 10 Euro Cent coin into the CMOS battery socket (or bridge the +/- contacts with a paper clip).
5. Wait ~ 15 seconds (the longer the better).
6. Remove the 10 Euro Cent coin (or the paper clip).
7. Insert the coin cell battery (CMOS battery).
8. Plug the power cord (on notebooks insert all accus).
9. Turn ON the computer, enter the BIOS-Setup and load the "Setup Defaults/Optimized Settings".
10. Save the BIOS settings and restart the computer.
11. No success? Repeat the steps 1-10.

Never turn on the mainboard during a CMOS-Reset!
It could cause a short circuit which would damage your mainboard!

First you should try the following:

1. Repeat the CMOS-Reset and increase the time to clear the CMOS from 15 seconds to a few minutes!
2. Check the voltage of your CMOS-Battery (~3V).
3. Remove all unnecessary additional hardware like e.g. TV-, Sound-, and Controller cards on your PC-Mainboard (not on Notebooks!). You only need CPU, RAM and Graphic adapter!
4. If you have more than 1 RAM module, remove the other (maybe one module is defective?).
5. Reset the Hardware table (DMI):
   the DMI (Desktop Management Interface) manages the integration of various hardware in a computer. Sometimes the DMI data table gets corrupted and you could try to reset it. So remove some hardware components like e.g. RAM and Graphic adapter and boot the computer for ~ 30-60 seconds. Then insert the hardware components and restart the computer. Note: if a working BIOS recognizes that some components are missed, the BIOS automatically scans for new hardware and refreshes the DMI hardware table.

Nothing works?

1. Check all cable connections.
2. Check all components and remove any existing dirt.
3. Replace CPU, Graphic adapter and the Power Supply Unit. Or use a second PC to check these components.
4. Check all Electrolytic Condensers.
   
   An electrolytic condenser is a cylindrical electronic component that stores electricity.
   
   The cylinder is mostly green, blue or black, and has a pressed aluminum cover.
   On older mainboards, it often happened, that the condensers get faulty. In most cases you can identify a defective condenser, when fluid leaked out of the cover, or when the cylinder itself is arched.