@Wibi 

Cmos Battery bad and power distribution board bad and power supply bad.

Same issue occurred with mine years ago. Here you go, found this: 

https://www.dell.com/support/manuals/en-us/alienware-area51-r4/alienwarearea51r5_servicemanual/clearing-cmos-settings?guid=guid-b43d3ab9-33fc-484d-9f4a-f05d5d088935&lang=en-us

You’re going to have to do this step a lot unless you just change out the psu. The link is for a separate system but it’s basically the same steps. Use the link to help you pinpoint the diagram for the X51 R2. 

@Wibi 

@Prometheus_ 

Rome Tech Alienware Andromeda CMOS Battery B09FTQ1H8N

x51-r2_owners manual

Dell Alienware X51 R2
Regulatory Model D05S
Regulatory Type D05S002

Does it matter wich cmo's battery i buy?

Im from Belgium and they dont sell that one here :s

I found this do: https://www.amazon.de/-/nl/dp/B08QG6S8NV/ref=sr_1_1?__mk_nl_NL=%C3%85M%C3%85%C5%BD%C3%95%C3%91&crid=10DZ425TEUK4Y&keywords=rome+tech+cmos+battery+for+dell+alienware+x51+r2&qid=1648307148&sprefix=rome+tech+cmos+bios+battery+for+dell+alienware+x51+r2%2Caps%2C211&sr=8-1

Is this a frequent problem with Alienware x51 R pc's?

@Wibi 

@Prometheus_ 

Yes the yellow light of death is VERY Common.

https://www.youtube.com/watch?v=l4Ru0EYZpuE

Whatever you do LOOK CAREFULLY at the OLD battery as far as the connector.

If the new battery is NOT the same cut the wires off the old one and solder them to the new battery wires and cover with heat shrink or tape.

https://www.youtube.com/watch?v=5DdAZrTZjEM

Thank you! im ordering a CMOS battery soon

So i will give a update ones it arrives!

Either way do not throw away the old battery because you may need the connector and wires to make a compatible hack.  Red wire goes to + and black wire goes to - 

https://www.youtube.com/watch?v=OPJ4r-f5olU

Fix your X51

https://www.youtube.com/watch?v=ypNG8w6AmTk

Battery is not visible without removing motherboard

Replaced the cmo's battery it sadly dident fix it

wrong polarity on the battery or one or more bad ram sticks.