What causes the 4 beeps with red flashing power-on led light on hp compaq dc 5100 sff

[Radevic1]

I have just replaced the power unit of the above stated pc. (Hp Compaq dc 5100 SFF). In my attempt to power up the pc, as soon as I connect the power cable to the power unit, the power-on led light close to the power button will immediately change from green to red flashing light followed with 4 beeps. I have cleaned the memory and its slots without any change from the behaviour of pc hence I am seeking the assistance of the Community to solve this problem.

[Rainbow23]

Hi @RadevicSpicer,

Welcome to HP Support Community. 
Thank you for posting your query, I will be glad to help you.

I see that you are facing booting issues with your HP Notebook PC.

I suggest that you run an extensive system test by following the steps listed below:
1. Hold the power button for at least five seconds to turn off the computer.
2. Turn on the computer and immediately press Esc repeatedly, about once every second. 
3. When the menu appears, press the F2 key.
4. From the main menu, click System Test and then click on Extensive Test.
5. Click Run once.
You may also refer to this weblink https://support.hp.com/lt-en/document/ish_2854458-2733239-16 

Let me know if this helps.
Take care and have a nice day.

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