I am looking at at Dell Optiplex GX150 that I was told would just randomly shut off while in use. When I started to look at it I was able to get it to boot and get to Windows desktop. I shut down the machine to look at the internal configuration, since then I haven't been able to get the machine to do anything. When the power button is pressed certain items internally can be heard starting, but the power light stays a solid amber color and the monitor has nothing displayed. I think the power supply might have just finally crapped out on it, just seeing if anyone had any other ideas.

The moment I began reading your post I thought PSU. Looks like you need to replace it to me. Be careful as some of the older Optiplex pc's had PSU's that were wired differently to the norm. If you should need it, I have all the wiring info to allow a standard PSU to be modified.

You know, I have my concerns with this whole setup. I went to dell to find a compatible psu by entering the service tag on the machine, but doing a little more research I found that the motherboard in this machine has both a 20-pin power connector and a 4-pin connector, yet the replacement psu doesn't have the 4-pin power connector. Do you think this is a problem to not have the 4-pin connector? apparently a psu has gone out prior to this one and someone replaced with a different model as it doesn't fit properly and has a 4-pin power connector in use.

If the mobo has the socket for the 4 pin then it is undoubtedly needed.

Can you take and post a photo of the psu plug as it is at the mo?

I will just add that psu's are my particular specialty.

The current PSU is a 160W Dell Model # PS-5161-1D1; like I said I don't believe this is the one that came with the machine. Hope this helps.

Hmm, it's a very awkward shape. It does look as if it is wired the same way as a standard PSU but you will never fit an ATX form factor one in there.
If you have a Digital volt meter I can give you instructions on how to test the PSU. It is still possible that the fault lies on the mobo so you need to know one way or the other.

the 4 pin plug is referred as the P4 plug and it needs to be plugged into the board or it will not boot,fans will run but that's it .
I buy and sell computers get most of them from this site ,I have owned a lot of this type of computer ,the psu in yours looks like the right one ,may not be original!
I live in Canada ,you can get the whole desktop unit for 68.00 + shipping .
http://www.vfxweb.com/index.php?productid=11516

Actually I just got the PSU tested and everything seems to be fine with the power supply. So I guess the motherboard is the problem but is there any way to test it to make sure it is faulty before ordering a replacement.

The thing is, to fully test a PSU you need to do on and off load testing.
I have come across a fair few PSU's that appear fine with no load on them but then break down when loaded.

Ok then so what do I need to do to get that accomplished?