Dell Optiplex 5040 SFF won't POST...

Hi all. I have an Optiplex 5040 SFF, Motherboard T7D40 that was purchased in May 2017. A couple of weeks back I was using it when I heard what sounded like a tick, or click, followed by my screen going blank and the monitor reporting that there was no video signal. The fans were still running. Holding the power button down shut the PC down. The button restarted it, but the fans kept running at full speed and it never completed the POST. This time the power switch would not shut it down and I had to unplug it. Then it would immediately start each time the power plug was plugged in but would never give/make any error indication and would just continue running the fans at full speed until unplugged. I can't remember if the power button was lit or blinking but it produced no audible beep code.

I'm not a tech and don't have spare parts. The PC is pretty nice and meets needs so I figured $30-$50 per step wouldn't be unreasonable and began a parts replacement process, starting with the simplest and least expensive.

- The PSU was $40, arrived in a couple of days and -- made no difference.

- I found an identical mobo replacement on eBay that the seller said was pulled from a working machine for $39. This time it still came on as soon as it was plugged in but now it also produces a one-time beep code of 1-3-2 -- a new symptom with this mobo. The power button is blinking amber. So, I did get a change in symptoms by changing the mobo but it still will not post.

Beep code 1-3-2 is a curiosity in itself. Dell documentation says they haven't used 3 number codes since 2005 except for 1-3-2 which is a "No memory failure" code. I get the same error when the sticks are out -- that would be correct -- as when they are in -- strange that both sticks would fail simultaneously.

- As I said, removing both sticks of RAM makes no difference in the symptoms and neither does putting one in at a time in each of the 4 slots.

- I've reset the CMOS and I've disconnected everything but just the mobo and the symptoms remain the same. Tomorrow I'll replace the CR2032 coin battery but this one is reading 3.1 VDC unloaded so I don't expect anything from that, other than a fresh battery.

Previously I tried to test the on/off button by checking for continuity across the switch when depressed but I was by myself and my lead terminals were too big and I was never able to be confident that I was actually making good contact with them when depressing the switch -- just needed at least one more arm. Tomorrow I'll get some help and get a good test of the switch. I'll also try shorting pins 1-4 on the mobo header to simulate a known working switch.

That leaves the CPU (Intel i5-6500) which could be a single component that might be responsible for a failure to find the RAM, and the RAM itself, 8GB (2X4G),1600, DDR3L. I suppose there's a greater likelihood of a single component failure (CPU) than two sticks of RAM failing but, as luck would have it, it's also the most expensive replacement part.  Plus, in lots and lots of years, I have never ever once had a CPU actually fail.  I've had them overheat and throttle back/off but never failed.  Perhaps this will be my first...

After reading this, assuming that I haven't already put you to sleep, does anyone have an idea of what I might do differently from what I have done, look at something else, or a way of getting a little more confident of what the next step should be?

Thanks in advance for whatever help you can offer,

Howard
Redmond, WA