Desktop PC not turning on, blinking green LED on motherboard

[Wrichik Basu]

Our Windows 7 32-bit desktop PC is fitted with an Asus P7H55-M LX motherboard. It was working fine till yesterday evening, but from this morning, it won't turn on. (By this, I mean that it is not even starting the boot; the fan on the processor won't run.)

My first guess was the PSU. So I unplugged the 24-pin ATX connector, and connected the green and one black terminal. The SMPS turned on properly with its fan running.

I checked the voltage of the different terminals with my multimeter. The results were as follows:

Wire colour	Potential difference w.r.t. COM (black wire) (in V)
Yellow​	11.92 to 12.0​
Orange​	3.22 to 3.3​
Red​	~ 5.0​
Blue​	-12.0 to -11.95​

On checking with the internet, the results seem acceptable, so the SMPS is working fine.

There is a green LED on the motherboard labelled "F_PANEL". In the past, when the SMPS was connected to the AC mains, this light would glow steadily even before we switched on the PC using the power button. Now, however, this LED is blinking:

I have tried shorting the power button terminals on the motherboard, and also tried to switch on the PC without the RAM. Nothing works.

I need some help in figuring out and solving the problem. If the PSU is fine, then is the motherboard faulty? How can I confirm that it is faulty?

 

[scottdave]

My initial guess would be F_PANEL stands for "Front Panel". I did a search for "F_PANEL Motherboard" and came up with a bunch of videos and links about connecting the motherboard to the front panel.

Perhaps there is a problem with the switch on the front panel? Or the connection to the front panel.

Perhaps you can search to find results which pertains to your specific scenario.

 

[russ_watters]

Find the blink code in the motherboard manual.

 

[MikeeMiracle]

Unplug everything bar the CPU/Memory and Graphics card if it has one. These are the essential items needed. If it then turns on you know the problem is with one of your periphery devices.

Unfortunately it's usually a case of swaping out different components with a working PC to narrow down the problem.

Higher end motherboards tend to have displays on them that display numbers depending on how far they are through the boot sequence, you woudl need to cross reference the number against the user manual for the motherboard.

 

[Rive]

So I unplugged the 24-pin ATX connector

Do a second voltage measurement with the PSU plugged on the motherboard. You can access the terminals from the top side of the connector.

What that blinking tells me about is not an error code but a faulty connection or a faulty PSU (likely the standby 5V).

 

[Wrichik Basu]

Perhaps there is a problem with the switch on the front panel? Or the connection to the front panel.

I thought of that possibility and tested as follows: removed all front panel accessories connection (only USB ports are available in the front panel) and shorted the power button terminals on the motherboard. Still unsuccessful.

Find the blink code in the motherboard manual.

The manual of the motherboard is available here:
https://www.asus.com/Motherboards/P7H55M_LX/HelpDesk_Manual/
Unfortunately I could not find any LED blinking codes there.

Unplug everything bar the CPU/Memory and Graphics card if it has one. These are the essential items needed. If it then turns on you know the problem is with one of your periphery devices.

Tried that already. Didn't help.

Do a second voltage measurement with the PSU plugged on the motherboard. You can access the terminals from the top side of the connector.

The PSU doesn't start if I connect it to the motherboard. (But it starts properly if I short those two terminals.)

 

[MikeeMiracle]

So it's either motherboard / cpu or memory then. You can try removing all the memory and firing it up with 1 stick at a time to determine if any stop it booting. If that doesn't work then it's either motherboard or CPU. In all honesty though computers are available for a few hundred pounds these days, I am not sure it's worth repairing if it's old enough to be running Windows 7. Your better of putting the money towards a new computer.

 

[Rive]

The PSU doesn't start if I connect it to the motherboard.

Check if the standby 5V is there and stable.

 

[MikeeMiracle]

I would be weary of focusing purely on the power side. These days the motherboard has constant power and it signals the PSU to turn on via a soft switch. If the motherboard does not find the CPU / Memory ready to come on due to a fault it simply won't send the signal to the PSU to power up properly. It does not mean there is a fault with the power circuitry itself.

 

[Wrichik Basu]

Check if the standby 5V is there and stable.

The purple wire gives 5.1V, and is stable.

These days the motherboard has constant power and it signals the PSU to turn on via a soft switch. If the motherboard does not find the CPU / Memory ready to come on due to a fault it simply won't send the signal to the PSU to power up properly. It does not mean there is a fault with the power circuitry itself.

Quite possible, but I detached almost all detachable peripherals from the motherboard (LAN card, front panel USB connectors, RAM), but even then it does not start up, and the LED keeps blinking.

 

[MikeeMiracle]

I'm not sure it will start up without any RAM connected, it's a core component.

 

[Wrichik Basu]

I'm not sure it will start up without any RAM connected, it's a core component.

I tried with the RAM connected and everything else disconnected. Even then it didn't start up.

 

[Rive]

The purple wire gives 5.1V, and is stable.

It was measured while the LED blinking? If yes, then I think it'll be a faulty motherboard. At this point some spare components for swapping would come handy.

 

[Wrichik Basu]

It was measured while the LED blinking? If yes, then I think it'll be a faulty motherboard. At this point some spare components for swapping would come handy.

One of my father's colleagues has agreed to take a look at the motherboard. He is a certified guy in these fields. Let's see what he says.

 

[Tom.G]

In the first post you stated that the supply came on for testing when the Green and Black wires were jumpered on the 24-pin connector.

Try jumpering those pins with the 24-pin connected to the motherboard. If something smokes, you've found the problem.

It could still be either the supply or the motherboard that is bad.

The supply has a current limit circuit built-in so it shuts down on an overload. If there is a short on the motherboard, the supply will immediately shut down.

The supply also checks that each of the output voltages is within the proper range. If all are good the PowerGood (PWR_OK) signal is sent to the motherboard, 24 pin connector, pin 8, Gray wire, +5V if OK. Without it, nothing powers up.

Here is a schematic of a 200W ATX power supply.
The power-on (PS_ON# active low) input is at the left edge, about 1/3 up from the bottom.
The POWERGOOD output is lower center, about 1/3 up from the bottom.
Line input is at top left, and outputs along right edge.
PowerGood sensing is at left center (just above PS_ON) and labeled OVERVOLTAGE CIRCUIT.
5V_StandBy is at bottom left.
The center is taken up by the switching logic and drivers feeding the power transformer T3.
--

--

edit: corrected PS_ON# input to show it is normally hi, active low

Cheers,
Tom

 

[256bits]

the fan on the processor won't run.

Bad fan?
I had that problem once.
Thing wouldn't boot because of the fan, or at least that is the area where I narrowed it down to as the most probable cause.
A laptop btw.
Maybe the boot testing is different for them than a desktop.

 

[scottdave]

We had an issue with a Mac, one time refusing to boot up. Finally, I started removing and reseating all of the cards and the RAM chips. It ended up after reseating the RAM, it booted right up.

 

[Vanadium 50]

You really want to buy a PSU tester if you want to test PSUs. Amazon has them starting at $8.

 

[hutchphd]

You have checked the CMOS battery?

 

[Wrichik Basu]

The supply also checks that each of the output voltages is within the proper range. If all are good the PowerGood (PWR_OK) signal is sent to the motherboard, 24 pin connector, pin 8, Gray wire, +5V if OK. Without it, nothing powers up.

It seems that the problem is with the PSU after all. After you said, I checked the gray wire, and it gave 40 mV. In addition, Dad's colleague has checked the motherboard and found it everything in working condition - from the CPU fan to the RAM.

Bad fan?

I don't think so. See above.

You have checked the CMOS battery?

Yes, it gives 3.09 V.

So, I think we can conclude that the problem is with the SMPS.

 

[hutchphd]

Can you get your hands on another supply? Its just two plugs and you can test it in two minutes. The flickering LED certainly points to it.

 

[Wrichik Basu]

Can you get your hands on another supply? Its just two plugs and you can test it in two minutes. The flickering LED certainly points to it.

It will take some time before I can lay my hands on a new PSU, but, as I said before, Dad's colleague found no problem when he tested the motherboard. (He used a different PSU.) It is, therefore, clear that the problem is due to the faulty PSU at our home.

 

[hutchphd]

You have no access to a junk computer somewhere...The power supply need not fit inside your box for the test (of course the plugs must match). If it works you know for sure! No fixit shops that could lend you one?

 

[Wrichik Basu]

No fixit shops that could lend you one?

Unfortunately we do not have any computer repair shops nearby that can lend me a PSU.

 

[russ_watters]

The manual of the motherboard is available here:
https://www.asus.com/Motherboards/P7H55M_LX/HelpDesk_Manual/
Unfortunately I could not find any LED blinking codes there.

Odd -- the LED is mentioned for normal operation, but that's it.

But google tells me that flashing green is a pretty standard (including for ASUS) "your psu is bad" message:
https://www.google.com/search?q=mot...e..69i57j33.3886j0j4&sourceid=chrome&ie=UTF-8

 

[Wrichik Basu]

Update:

We bought a SMPS from Amazon India: https://www.amazon.com/gp/product/B07YVWMV5D/?tag=pfamazon01-20. It was a bit costly, but other low cost PSUs had very bad reviews (getting burnt within two days, shorting the motherboard, etc.), so we decided to buy this one. It arrived today, and upon plugging in, the PC has sprung back to life!



Thanks everyone for your advice/help, especially @Tom.G, who pointed out that even if all voltages are up to the mark, the PWR_OK pin needs to give 5V to indicate the the PSU is working properly.