Hello!  I would unplug everything go into the computer and unplug any cards you have like graphics card and network cards etc. Also unplug your HDD and Optical Drives and plug them all back in and see if that clears it.  I looked at the manuals you did and just combined 1,2 and just 4.  

If I hazard a guess, I'd say it's the video card based on code 1,2,4.

1,2 = A possible expansion card failure has occurred.

2,4 = A possible graphics card failure has occurred.

I suspect you may be seeing a combo of both of those LED codes. So first thing is to reseat the video card in its slot.

And if this PC sat around for a long time, unplugged, with a defective PSU, then you probably want to replace the motherboard battery right away, if nothing else than to rule it out as contributing to the problem. It's a CR2032 3-volt lithium ion battery, ~$2 at discount stores.

Duracell DL2032 also works.  They keep them in the *** section of grocery stores because blood sugar meters use the same batteries.  Even though they say medcal device they work on computers just fine.

Hmm.. Didn't think of the BIOS battery issue.