Hello there!
Old thread, I know.. but I'm facing similar problem and I might have found what's causing this strange behavior at my new computer.
I have heard the "long-beep+3-short-beeps" when booting up with the monitor turned OFF.Computer boots up with no "long-beep+3-short-beeps" if monitor is turned ON before post boot.Let me explain better what I think is happening here...
I have bought a new motherboard and CPU:
ASUS ROG Z590-I motherboard and
Intel 11700K CPU.
After having installed everything and updated the motherboard bios to the last version, I made some hardware tests so to check if everything was working good, and while doing so, at some point, I heard the "long-beep+3-short-beeps" !!!
When I first heard it, it was while testing/trying to boot with the CPU integrated graphic card.
Let me step back a little.
I have always thought that the internal graphic card is always active (or at least active when selecting the default bios settings). As I was indeed in default bios settings when doing these tests, I was confident that the internal graphic was always turned on (just to be clear).
I then later understood that in my particular new motherboard this is working differently: the CPU internal graphic card is on/active ONLY when there is no PCIe GPU installed in the system, or better said,
when there is a PCIe GPU installed onto the motherboard, the CPU integrated graphic card is disabled/turned OFF, such that it is not even present/visible in Device Manager within Windows 10.The first time I turned my system on with my monitor attached to the motherboard HDMI port, I heard the "long-beep+3-short-beeps", and just a few seconds later I realized that my monitor was turned OFF. Turning it ON was not showing up nothing, so I had to reset the system by pressing the reset switch on the case.
When the computer booted up again (still with the CPU internal graphic card) the monitor was ON since before the computer restart, and this time everything went smooth.
So for booting up with the CPU integrated graphic card I must turn ON my monitor before motherboard post boot, otherwise it ends up with the "long-beep+3-short-beeps" forcing me to reset the computer to boot up again correctly.Later I have mounted the PCIe GPU onto the motherboard and closed the case, and at first boot I still heard the "long-beep+3-short-beeps" !!!
Guess what?? My monitor was turned OFF !!! (I have probably turned it off as a reflect action that I do every time I turn off the computer).
I then turned the monitor ON and guess what??? I was able to see my Windows 10 login screen!!! (probably when booting up with a PCIe GPU installed and a turned OFF monitor would end up with hearing the "long-beep+3-short-beeps" but with the ability of seeing something on screen if turning ON the monitor, to the contrary of what happens if booting up using the CPU internal graphic card where a reboot is necessary).
I then rebooted correctly my computer plus turning it off and turning it on again (made both tests), both with the monitor kept turned ON, and the result was NO "long-beep+3-short-beeps".
So for what I know now, I can stand that for the motherboard to boot up correctly, it needs to feel a turned ON monitor plugged into the PCIe GPU or CPU internal graphic card, otherwise it will end up advising some sort of anomaly using the "long-beep+3-short-beeps", with or without the ability to reach (respectfully) the Windows 10 login screen.Personally speaking, it sounds a bit strange, and I have not found any official confirmation about this.
I'll contact ASUS costumer support opening a ticket asking them about this.
I'll keep this thread updated.
Thank you for reading.