How to Clear a CMOS

Clearing Your CMOS

In the course of troubleshooting customer issues, our support technicians often suggest clearing a system’s CMOS, with the goal of remedying corrupt BIOS settings. CMOS corruption is generally caused by power fluctuations, poor grounding, or a bad CMOS battery, and is one of the most fixable issues our customers encounter.

Clearing the CMOS resets low-level BIOS settings responsible for telling the mainboard how to start up, where to look for the operating system, and how to behave once the operating system takes over. Corrupt settings can cause undesirable operation, or even a failure to start, or POST.

Unplug the Battery

To clear your system’s CMOS settings, you’ll need to start by disconnecting the system from power. After that, you will need access to the mainboard and remove the small coin battery mounted somewhere on the board.

In most cases, there is also a jumper you can move to speed up the clearing process. It is usually identifiable by the label ‘JBAT’ silkscreened nearby on the board, and occupies a 3-pin header, with one pin uncovered. The jumper will need to be moved from pins 1 and 2 to pins 2 and 3, and then the system should be left sitting for approximately 30 seconds.

After waiting the appropriate amount of time, reverse the process by moving the jumper back, replacing the battery, and reconnecting power. Then, you can power the system back on to see the results.

You’ll see a message, like we have below, declaring that the CMOS has been cleared, the checksum is incorrect, or that the battery has failed or been removed. In every case, you’ll need to re-enter the BIOS to reconfigure it for the behaviors you desire, since the BIOS has been restored to factory defaults.