When a computer fails to POST it can be for a variety of reasons, whether it’s bad hardware or a setting change in the BIOS. Sometimes the only way to get back into the system is by clearing the CMOS and resetting all BIOS settings to default.
CMOS refers to the small battery that sits on the motherboard and supplies a small amount of power to it when the system is powered down or unplugged, and is responsible for making sure the BIOS keeps the correct time and date as well as any overclock settings, among other things. The battery is about the size of a quarter and is the same type that is used in wrist watches. The CMOS is necessary since the part of the motherboard that stores BIOS settings is what’s called volatile memory, meaning it loses its contents when it loses power. The CMOS battery supplies just enough power to prevent that from happening, but it doesn’t last forever and you will eventually start to see messages saying the BIOS was reset to factory defaults. This means you will need to replace the battery, and you can find a guide on how to do that here.
In a situation where your system isn’t POSTing clearing the CMOS can sometimes enable you to get back into the system for further troubleshooting. There are a few ways to go about this, depending on your motherboard. If you have a newer and fairly high-end motherboard, they often have a dedicated “Clear CMOS” button on either the rear I/O near the USB ports or on the motherboard itself, usually along the edges of the board. To use it, power the system on and then press the reset button. The system may power up and down a few times but if successful will bring you to a screen saying the BIOS defaults have been loaded and will prompt you to enter the BIOS. No matter how you clear the CMOS, you will see this screen if it works. The next method is a little trickier but still fairly simple. All motherboards have small groups of pins sticking out called jumpers that serve various functions. There is a special pair of jumpers that are dedicated to clearing the CMOS by connecting the two with a screwdriver. On most boards it will be labeled “CLRTC” or “clear real time clock”, another name for the CMOS. Where it’s located on the motherboard will depend on the model, but will generally be near the bottom edge. If you can’t find it, the motherboard manual will show its location. Once you’ve found it, make sure the system is powered off and get a small screwdriver ready (it can be either Philips or a flathead). Power the system on and place the screwdriver between the 2 jumpers so that it’s touching both at the same time. If done correctly the system will power up and down a few times before taking you to a screen saying the BIOS has been reset. The last method can potentially be more difficult, depending on the layout of your motherboard. The CMOS can also be cleared by physically removing the battery from the motherboard. First, unplug everything from the computer and then locate the CMOS battery. Depending on your motherboard it may not be immediately visible and could be under the video card or under a part of the motherboard. If you’re not sure, check the motherboard manual. Once you’ve found it, it’s held in a circular plastic container with a metal tab on one side. Using a small screwdriver press the tab and the battery should pop out. Now leave it out for about 30 minutes to let all power from the board drain out. Put the battery back in and reconnect all the cables and power the system back on. If it worked, you’ll see a message saying the BIOS has been reset.
Some situations where clearing the CMOS will not work may include a bad stick of RAM, a bad video card, bad motherboard or bad CPU.