CMOS Battery

Signs the CMOS battery might need replacing:

  • BIOS UI stops responding to keyboard input after a few seconds
  • Getting into the BIOS UI is tricky/flaky
  • “CMOS checksum error” on boot

Other things I tried that didn’t help:

  • Reset the CMOS by shorting the CLR_CMOS pins.
  • Unplug all RAM and try one stick at a time.
  • Unplug all hard drives
  • Different keyboard/USB port combinations.

Replacing the physical CMOS battery (probably 5+ years old at this point) fixed things completely.