Windows 10: Power supply not On when I press Power Button

I've been facing an issue with my Dell OptiPlex CPU for the past few days, and I'm in need of some expert guidance to find a permanent solution.The problem I'm encountering is that my Dell OptiPlex CPU is not turning on. When I press the power button, the light on the button blinks rapidly, but the system fails to boot. Interestingly, I've noticed that when I press the Power Supply Diagnose button near to power cable port and the CPU Power button simultaneously, the system does turn on. However, having to go through this process repeatedly has become quite tiresome.I am reaching out to thi

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When I press the power button sleep, hibernated, or power down don't work my screen goes off but not my pc

Hi,

Thank you for the update.

To assist you better, I would like to know when exactly you had received this error. Are you able to boot the computer?

The following steps detail how to change what happens when the power button is pressed.

By default the power button will tell the computer to shut down, if it is not set to shut down, please follow the below steps.

• In the Windows search box enter ‘Power Options’ and click on it when it appears in the list.
• On the left of the window, click on the ‘Choose what power buttons do’ link
• Under ‘When I press the power button’ choose the option you want to change it to
  Shut Down.
• Click ‘Save changes’ to save the changes.

If the issue persists, I suggest you to run the Power troubleshooter and check if it helps.

Follow these steps to run Power troubleshooter:

1. Type troubleshooting in the search bar.

2. Select Troubleshooting.

3. Select View all on the top left corner.

4. Click on Power.

5. Follow the on-screen instructions to run the troubleshooter.

6. Check if issue persists.

After performing the power troubleshooter, please perform the steps that has been provided above.

Hope the information helps. Let us know if you need further assistance. We will be happy to help.

Thank you.

 

Help with power supply not working

Also look at this to test the PSU
How to Check a Power Supply: 12 Steps (with Pictures) - wikiHow

 

When I press and hold power button

Hold the power button for 15 seconds or so. When you press the power button for a second or two, it will go into Sleep Mode. Hold it longer and it will shut off.
Boot into Windows go to Search and type power options and press enter and change what the power button does and change it to Shut Down. How to customize power button action when pressed on Windows 10 | Windows Central