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There is a similar post about this issue, but mine is slightly different.

I've just built another computer and I haven't yet connected the power and reset buttons to the motherboard, but I have connected the power supply to the motherboard. However, when I plug the power supply in and flip the switch on the back, my computer actually tries to boot. It doesn't automatically turn off (like the post linked to above), instead I get some BOIS beeps (one long, two short), which according to my BIOS means I have a video error (which could be seeing as I haven't plugged a screen in yet).

What could be causing the instant boot?

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up vote 2 down vote accepted

Is this a new motherboard? As in... just purchased, never used by anyone before? Because default BIOS settings would not be set to power up automatically upon receiving power. Regardless of whether a BIOS could be set to do this, it would not be set to do this out of the factory. You would have to go into the BIOS and change the behavior to do this yourself.

Of course, if this is NOT a new motherboard, then all bets are off, since we don't know what the previous owner set it to.

You would NOT be getting BIOS beeps just because you do not have a monitor attached. Go ahead. Attach a monitor. See for yourself.

It sounds more likely that there is an issue with the motherboard. A lower probability is that there is a problem with the power supply. You can eliminate that by swapping it out with another one (the power supply).

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Its a motherboard from a store-built pc (a gateway I believe). The power supply is new. Yeah I didn't think the video stuff was because I didn't have a monitor attached, but the documentation does say the beeps are video related. – n0pe Jun 2 '12 at 17:25
@MaxMackie I'm not doubting that the beeps are related to video. I'm just saying that you won't get those error beeps just because a monitor isn't connected. If there is a video related issue, you would get those beeps regardless of whether or not a monitor was connected. – Bon Gart Jun 2 '12 at 17:28
I'll see if maybe the motherboard is expecting a video card. After all, this was a pre-built computer so it might have been hardcoded that way. – n0pe Jun 2 '12 at 17:37
@MaxMackie examine the motherboard for the CMOS clear jumper, and use it to return the BIOS to defaults. If the motherboard has onboard video, clearing any changes from the default BIOS settings should return it to functionality. If the motherboard doesn't have onboard video AND there is no video card present... then yes, it needs a video card. – Bon Gart Jun 2 '12 at 18:02
I added the graphics card that originally came with the computer, and it works. Thanks – n0pe Jun 2 '12 at 18:35

First: A missing monitor/screen would not cause warning beeps from the BIOS

As to your actual question: Some motherboards can be configured what to do after power appears. This can be

  • Stay off
  • Resume previous state
  • Power up.

These options are mostly found on server boards, but I also found them on some non server motherboards.

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I know, it's an old post, but in case anybody new is reading it: it may also be because your cmos battery is dead.

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