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[ The PC Guide | Troubleshooting and Repair Guide | The Troubleshooting Expert | Troubleshooting Specific Components | Troubleshooting the Motherboard and System Devices | CMOS Memory or Real-Time Clock ]

I need to clear the CMOS memory (due to a corrupted BIOS, lost password or other problem) but do not know how to do this

Explanation: You need to clear the CMOS memory but aren't sure how to do this. It is sometimes necessary to clear the CMOS due to a lost BIOS password, corruption of the CMOS memory, or because you set the BIOS settings to values incompatible with your hardware and now you cannot boot the PC far enough to get into the settings and reset them (this rarely happens, fortunately).

Diagnosis: How easily you can clear the memory depends on the design of your motherboard. In some cases it can be easy to do but in other cases very difficult.

Warning: Erasing the CMOS memory will cause you to lose all settings in the BIOS. Make sure that you only do this if it is absolutely necessary. Basically, you should only do this if you can't get into the BIOS setup program due to hardware problems or a lost password.

Recommendation:

  • Turn off the PC. Hold down the {Insert} key and then turn the PC on and wait for it to boot. On some PCs, this will clear and reset the CMOS memory for you. (On most PCs it will not work, so don't be discouraged.)
  • Try the same thing with the {Delete} key. Again, it usually won't work.
  • Look in your motherboard or system documentation for any evidence of a CMOS clear jumper. This is a jumper on the motherboard that can be used to clear the CMOS memory; many newer motherboards have them. Follow the instructions for its use as described in the documentation; usually this means opening the PC, changing the jumper to a special setting, and then booting the PC. The CMOS memory will be cleared. Then you power the PC down and put the jumper back to its previous position. If it doesn't work properly when you try it, look here.
  • If you do not have a CMOS clear jumper, your next option is to try disconnecting the CMOS battery. This is easy to do if the battery on the motherboard is removable or user-replaceable. If you see on the motherboard what looks like a flat round wristwatch or calculator battery in a holder, that's it. Some older motherboards use batteries that sit off the motherboard and connect with a wire. If the battery can be disconnected, then disconnect or remove it. Wait for about two hours (you may need to vary the amount of time; if two hours isn't enough, try leaving it overnight) and then plug it back in, and the CMOS should be cleared and reset.
  • On some systems, the CMOS battery is integrated within the BIOS chip. You may have success with removing the chip for a few minutes and then replacing it. Just be very careful to take anti-static precautions.
  • Your motherboard may have a battery that is soldered to the motherboard. You may not see a battery on the motherboard at all; if this is the case then your motherboard probably uses a battery that is integrated into the real-time clock chip (or else, you weren't looking closely enough :^) ). Unfortunately, on a motherboard without a removable battery and with no CMOS clear jumper, clearing the CMOS memory is difficult to do. At this point you should contact your manufacturer for technical support.

Warning: Some people will recommend shorting the leads of the battery to clear the CMOS memory. I do not recommend this procedure, because shorting things on the motherboard is just generally a dangerous thing to do. Even removing the CMOS chip has the potential for problems. It really is best to contact the manufacturer of the motherboard or PC you are using in this situation.


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