Trouble Signs

• Public domain
  
  The computer is losing time and date settings. You're noticing messages such as: "CMOS Read Error," "CMOS checksum error," "load optimized defaults?" or "Boot sector not found."
  
  Other signs are plug-and-play devices not being recognized or a floppy disk or hard drive not found on boot sequence.
  
  

Causes

• The CMOS battery is low in power. The battery will drain if the system has been in storage for an extended period.
  
  Debris or obstruction, such as dust on the contact points or battery, can cause failure. A short in the system board or expansion card is another cause.
  
  

Considerations

• There are viruses that can overwrite BIOS settings. Such a virus can mimic the messages on the video screen to trick you, but this does not happen very much nowadays.
  
  A faulty mainboard can cause corruption that looks like CMOS issues.
  
  An insufficient power supply unit can cause expansion boards and i/o cards, such as video cards, to malfunction. Sometimes this may be mistaken for a bad CMOS.
  
  

Maintenance

• Avoid long periods with the computer being in a powered down state. Leave your computer running, but turn off the monitor. Enabling power saving modes for the drives and monitor can accomplish this.
  
  The temperature inside of the cabinet has a bearing on the battery. Change the battery at the two-year point.
  
  If there is a hint of CMOS issues when troubleshooting, change the battery.
  
  The voltage of the battery should not fall below 2.5 volts. Check the contacts points with a voltmeter.
  
  

Solution

• Change the CMOS battery. Take the old CMOS battery with you when going to purchase a new one. The battery or cell has a number. The battery usually is numbered "CR2032." These batteries are inexpensive.
  
  

Replacement

• Print out or write down your BIOS settings. Power down and open your computer, and locate the coin-sized battery, like the one in the picture. The battery is in an easy-to-access slot on the main board.
  
  Remove the old battery. Wipe the contacts gently with a dry cotton swab to clean it if necessary. Make sure there are no fingerprint smudges on the new battery.
  
  Put the battery in its holder. Sometimes there is a retaining clip that gives a positive click on insertion.
  
  Replace the cover and start up your computer. It should boot with no issues. Go into set-up and put back your settings. Reboot and you're done.