If your power supply does not appear to have any problems after testing it with the multimeter, check all of the power connectors to make sure they are properly plugged into the devices.

Power on the computer. The multimeter should read between 11 and 12 volts. If it's too high or too low, the power supply is failing and needs to be replaced. Test pin 12 as well.

If you want to test the Power Supply Unit (PSU) of your Windows computer, use one of the methods mentioned in this post.

You can check the power supply on your PC by removing the side panel of its case, or by checking the manual that came with it.