P8CWS motherboard, i7-3770T CPU (this includes onboard graphics card), 32 GB DDR3 RAM, WD30EFRX 3 TB hard drive, Pico M4-ATX-HV 250 W power supply.

It began by the entire system seeming to pause for a few seconds while the mouse cursor still moved normally. One day out of the blue the PC just powered off instantly and did not boot after that.

Eliminating all hardware except one RAM module, the CPU, the motherboard and the PSU, still the same: the motherboard's DIAG_CPU LED lights up while all fans are turning normally and no beep at all is heard.

The manual says this about the DIAG_CPU LED:

Is there anything else to look at to ensure it is the CPU that is faulty? Before returning the CPU to the shop under the 3 year warranty?

What if they claim it is my fault so the damage is not covered?

I take it that the system doesn't POST. At all? The last systems I had this happen on were my MSI x86 and an HP ML370 G4 server. The cpus were fine. I had to replace the system board/Mobo in both cases. Symptoms were like yours, fans OK, no beeps, no video. When the MSI mobo failed I just had a desktop display and everything had hung. Power cycled to nothing but fans. The ML370 has diag leds but didn't highlight anything. May be a toss up, which is easier/cheaper to replace first? Sounds like you didn't do anything to kill it so I'd take it back to the shop.

Good luck!

Play Bonny!

I'd measure all voltages under load before doing anything else. I'm sure you know ATX pinout. If not, Google.

The CPU is from a different shop than the motherboard and ram. And both were bought with mail order.

Should I take it to a repair place first, so maybe they can plug another CPU and see if it works, or another PSU and see if it works?

Is this a common CPU socket? Or are CPU's too expensive to keep in a repair shop for such diagnosing?

Are motherboards a more common source of failure than CPU's? Given the CPU fault LED is on, does it make unlikely it is the motherboard's fault?

By the way, NO hissing noise from motherboard.

If the thermal paste is too much or not enough on the CPU, does the CPU burn and get destroyed permanently, or just halt?

I figure you do not have a voltmeter. Try another PSU. Besides, your 250 W PSU may be sufficient ... or it may be too weak. It really is about how much power is drawn on 5 V, 3.3 V and 12 V. Even if the overall wattage is sufficient power rails may fail to provide sufficient power on specific voltages.

Try eliminating the RAM sticks first as that is more common. Plug in only one RAM stick at a time and see if it boots.

I think the PSU may be fine, I mean the fans are spinning, which means they have power. You could try a spare one if you have it. I agree that 250 W is on the very low end. If you were to add a graphics card, it would surely burn out. Also, accounting for capacitor aging, the PSU will not last more than a few years before burning out.

In 20 years of being a PC technician I have never seen a CPU failing. Unless the cooling was insufficient or the PSU generated an overvoltage.

Overheating of modern CPU doesn't occur anymore as they shut down automatically.

As advised before, check the PSU with a voltmeter.
For testing you really only need memory, the CPU and the mainboard. No disks, VGA or NIC.
Suspect the memory. So replace it by known good and use one at a time. Even if you use known good memory it might not be good.

I assume you are in a difficult situation and you don't have tons of spare parts. So I would advice you to take the CPU back to the shop and ask them to test it for you.

Odds are 90% for a defective MB, 10% for a defective CPU. You have to do something.

jlinkels

I'm with jlinkels... I'd still reckon the Mobo (which has probably only got a year's warranty?)
I did the PSU thing, I've now got a spare 450W PSU on the shelf which cost £30.00 odd, didn't fix my dead box but as the fans are running, the 12V should be OK. I assume the disks are spinning as well. The +5V should be OK as you have LEDs showing on the board. If the 3.3V for the CPU/Memory is missing, it's generated from the +5. The VCM is on the Mobo.

Taking it to the repair shop is going to cost you what? I'd be charging about £60.00 - £70:00 as a "homer" to swap the PSU to eliminate it, Plus the PSU cost if it fixed it Your Mobo is about £170.00 from what I can see, probably the biggest cost.
("homer" = home job, outwith normal work)

I think you have to bite the bullet and either claim warranty on the Mobo, if it's still covered, or buy a replacement.

Good luck!

Play Bonny!

Alright, did the ram swap procedure. None of the 4 modules makes a difference (connected at the appropriate socket for single-module 8 GB).

Measured the PSU voltages while connected to the mobo, here they are:

(looking towards the mobo)
3.40 ......... 0
12.56 ....... 5.16
12.57 ....... 5.16
5.15 ......... (no cable here)
4.97 ......... 0
0 .............. 0
5.16 ......... 0
0 .............. 0
5.15 ......... 0
0 .............. 0
3.4 ........... -12.38
3.4 ........... 3.40

Is it ok that there is no -5 V?

From at I can find out, the -5V supply was optional, (on pin 20) it was removed from the ATX spec as of ATX12V V2.01
I stand corrected about the 3.3V supply; pins 1,2,12 & 13.

Play Bonny!

So if the voltages are ok the power supply is almost certainly ok?

And it is the mobo that should be RMA'ed?

You've tried re-seating the CPU in its socket, right?

Put the cpu and ram on another motherboard powered with the psu, and they all work. So it's definitely the motherboard.

Too bad for an expensive motherboard that it died after just 1 year of use. Or maybe it is my fault? Psu output not quality enough?

Should I put a bigger psu next time despite the fact that the maximum power calculates to well below the 250 W of the dc-to-dc psu and say goodbye to efficient operation on caravan batteries?

The pc did pause some times in windows for a few seconds while the mouse cursor moved normally, as if the associated core ran normally while another core was taking a rest.

I think you're over rationalising the issue. It just happens sometimes. That's why you get a years warranty as probability theory says if it doesn't fail in this time you should be OK... There are always exceptions though. If the PSU was insufficient for the job I reckon you'd have been getting a noticeable amount of glitches, hangs and other weird stuff. Fit a bigger than needed PSU if it makes you feel happier. Just because it says it's a 450W PSU doesn't mean that that's what it'll be supplying. The Mobo, disks, etc will only pull what they need.

Play Bonny!

Check reviews before buying a new mobo. The quality of each manufacturer changes over time. So I mean while in the past I would buy ASUS mobos, today I wouldn't go near them.