My recently build system mainly contains the following components:

  • Mainboard: ASUS Z170-A
  • Processor: Intel I5-6600
  • PSU: BeQuiet S6 550W

(Until now I have been using the same PSU with a Gigabyte GA-P55-UD3 + i3-530 micro)

The motherboard has the following connections:

  • 24 and 8 Pin Power connectors
  • CPU and CPU-FAN
  • No memory (same effect with memory inserted)
  • Speaker (optional)


I switch on the PSU: PWR-LED on motherboard lights up. When I start the motherboard (the z170-a has a small pwr switch directly on the board), the CPU-FAN runs but no other LEDs light up (not even blink). It is the same effect when I do not connect the 8-Pin Power connector. The speaker (when connected) does not make any sound.

I think that something is broken or short-circuited.

Tests I have done:

  • PSU: connect old motherboard + old processor, works ok.
  • motherboard: uninstall motherboard and run outside the case, same result. There seems to be no visible damage on the motherboard .
  • CPU: No visible damage. Little connector feet on motherboard seem ok, too.

My Questions:

How can I know whether the problem is the CPU or the motherboard ?

When does the CPU-Diagnose LED indicate error? Should it light up when I remove the CPU?




With the help of ASUS and other forums I have found out:

  • The ASUS Diagnose LEDs only work if the CPU is installed.
  • In case of No-Post always connect the speaker to detect failure beep-codes. Those beep-codes can be explained with the manual, in my case the manual from the ASUS Z170-PRO, because the Z170-A manual does not include that table.

The order of failure probability is:

  1. Wrong installation
  2. PSU
  3. Motherboard
  4. CPU

My case was a little bit more complicated: My PSU had worked fine with my older Gigabyte motherboard. But on the new ASUS board I had No POST. Then after trying again on the older motherboard, that one also failed! But other than the ASUS board the Gigabyte board did provide continuos beeps. And the Gigabyte manual states:

Continuous short beeps: Power error

Then it was quite clear that the problem was the PSU. So now I have a new Power Supply and everything is working perfectly.

I hope this helps others facing similar problems.


Without wishing to offend you, have you checked and triple checked that the CPU fan is connected to the primary CPU fan connector and not a secondary optional connector?

I only suggest this as I've made this mistake a number of times when the motherboard hasn't had it clearly marked.

  • No offense at all :-) But would connecting the cpu-fan to the wrong connector reproduce my results? The CPU-fan is actually the only thing working. – JanCG Jan 11 '16 at 12:03
  • It would yes, if the CPU-fan is connected to the secondary connector and nothing is connected to the primary connector it will presume there is no cooling for the processor and therefore refuse to POST. – Corran Blyth Jan 11 '16 at 12:26

I'm facing a similar situation. All new bits fresh out of package:

  1. ASUS Z170-A
  2. Intel Pentium G4500 + Intel supplied fan
  3. Corsair 330R case
  4. Corsair CX750M PSU
  5. Corsair Vengance (really?) 2 X 4GB 2133MHz DDR4
  6. Crucial MX-200 250GB SSD.

I put all the labeled bits where the Z170-A manual said they should go, and the USB cable from the front which wasn't labeled but clearly went into a USB connector.

leftovers were a SATA-peripheral-power-like connector (something that plugs INTO the bandoleer cable that came with the PSU) 3 of what look like FAN connectors, but no fans connected to them that I can find.

I put the CPU Fan onto the MB's CPU fan connector, the upper left, light colored one, of the EIGHT connectors marked "(3)" in 1.2.3 "Motherboard layout" on the user's guide page 1-2. I put one case fan on CHA_FAN1 and one on CHA_FAN2.

Turn on the PSU and the green LED (Standby Power LED, item 1, page 1-34) lights up. Press the motherboard's Power On Button (item 17, page 1-2) and the CPU fan starts running. KVM dongle gets a green LED from USB connection.

But no output on the D-25 connector OR the HDMI connector. No flash of subsystem LEDs as the systems are autotested. Nuthin. No beeps. I put a pair of headphones on it, an amplified desktop speaker on the back pannel left channel, nothing.

Neither chassis fan turned on. So I swapped them around and ended up with them on the second CPU Fan connector, upper right, and on the CPU Opt (water pump) which BOTH start, if there's a fan on the left hand CPU fan connector, #3.

I disconnected SATA drive and power, no difference. I put the ram in A2 and B2, as the booklet shows. Maybe I'll move it.

If this is running for anyone I'd love to hear how they rigged the wiring. I've got the front panel reset button and white LED working

Thanks in advance!


  • In my case, all the individual pieces, chassis fans, SATA disk, RAM, power supply, and the processor,, worked A-OK in with other motherboards. I had a bad motherboard- just that simple. I built another box using a different motherboard and processor, transferred everything from the one that wasn't working. All good. Bought a new motherboard, moved the processor to it. Worked fine. – Bill IV Apr 6 '16 at 4:48

I had similar symptoms in my brand new Asus Z170-A. There was no beep at power on and the diagnostic LEDs got no further than the memory check. I was able to get beeps indicating no RAM if I removed all four memory sticks, but restoring any or all sticks killed the beep.

I discovered quite by accident that if I push on the CPU fan in the direction of the PCI sockets while I also turn on the power, the board would boot. This persisted even when the board was removed from the case and run bare on my bench. It also persisted when I removed and reinstalled the CPU.

Conclusion: bad board with an intermittent connection somewhere in the CPU area. As a design engineer, I know that the most common reason for this is a bad solder joint. I'm exchanging the board.

  • One other thing.. I tested whether it matters whether the CPU fan is installed on the right connector. If installed on the wrong connector, the board will still boot, but the bios reports an error message. So, the CPU fan needs to be on the right connector, but getting this wrong won't prevent beeps and display from working. – Gyrogearloose Jan 13 '17 at 3:53

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