The light on the back of the hp pavilion 520n is lit when it is plugged in, but the unit will not start up, nor do the fans run. Does this mean the power supply?

Usually the light will time out after the capacitor has drained. Your machine is getting a bit old , your PSU may be the problem. If you plug it in and push the on/off button and it blinks, fans spin and shuts down, it's most likely the PSU. Look for a in-line fuse on the Motherboard, if blown, replace. Look at the motherboard closely for damaged components, capacitors, etc.

The power is coming to System , but its not getting supplied to other hardware devices via motherboard.

The Problem can be with your motherboard.

Try removing the Auxilary port wire ,which is a small wire which is connected near the FAN , the wire comes from SMPS(power supplier) to the motherboard.

hope this helps

Try swapping out the ram 1 at a time, making sure there is one memory piece there while testing. If no! check hard drive for activity.

How to Replace a HP Pavilion dv3 Fan?

1. Follow your laptop owner's manual and assembly guide carefully you can download this from HP web site as you disassemble the laptop. Touch something metal periodically to remove static charge, and work on a large table with a bright light source.

2. Turn off the laptop and unplug it. Remove the main battery, CD/DVD drive and the hard drive. The hard drive is usually located in a slot on one side of the laptop. Remove the screw that holds the cover in place, then pull out the hard drive sled.

3. Turn the laptop over and remove the screws on the bottom that hold the keyboard in place. Flip the laptop over, open it, then lift the keyboard up and out. Unplug the flexible ribbon cable that connects the keyboard to the motherboard.

4. Remove the screws that hold the keyboard bezel and palm rest in place. Carefully pry loose any plastic snaps on the sides, then lift the bezel and palm rest up and out.

5. Unplug the HP Pavilion dv3 Fan Cooling Fan. Remove the screws that hold the heat sink and fan in place, then remove the entire unit from the laptop. Remove any thermal compound residue from the bottom of the heat sink and the CPU.

6. Apply a thin layer of thermal compound on the CPU, then replace the heat sink and fan. Replace the screws, plug the HP Pavilion dv3 Fan back in, then reassemble the laptop.

7. Plug the laptop in and turn it on. After you use it for a few minutes, you should hear the new fan running and feel warm air blowing from the vent on the side of the laptop. A HP Pavilion dv3 Fan does not run continuously, so don't be concerned if the HP Pavilion dv3 Fan shuts off periodically.

Difficult to say at this stage. If the fans on the PSU spin when the power button is pressed in and held, it would indicate that there is power to the PSU, but that does not rule out a faulty PSU

I'm suspicious about the fact you need to hold in the power button, which is normally a way of switching OFF the computer.

Might I suggest you try the following:

1.) As suggested before, remove the power switch connector to the Fpanel header on the board (should be labelled PW or pW switch). using two single pin connectors on the end of a piece of wire, connect them to the pins (does not matter in which order). Plug the computer in and touch the ends of the wires from the pins together briefly, if the computer boots, then the issue lies with the switch aseembly. If you are unhappy about doing this, you can use a multimeter set to the ohms scale and place one probe in each of the wires that go to the switch. Press the switch and hold it and see if you get a reading.You should get a reading and this will drop as soon as you let go the switch

2.) Replace the power supply and see if that works. This is the second cheapest option.

3.) If a new PSU does not work, then the issue must lie with the motherboard. I'm assuming you have already checked that the main multi cable is FIRMLY in the socket and the subsidiary 4 pin Black and Yellow cable is in the seperate socket

Probably a problem with the power module. Unplug it and lit is sit for a while then plug it in and see if it corrects the problem. If not just look at the back of the computer.
where the power cord enters,and see if the fan is going.
if there is any unlit led there. If it is unlit.
Or if the fan is not going., you may need a new power unit.
Remember that most computers have more than one fans. Look for the fan that cools the power unit, Not the one that cools the CPU or video cards.
They are lower down. and since these fans fan does not always have to run continually.,.

The fans are USUALLY controlled by the processor chip reading its internal temp and running fans as needed. Under the varying load during boot, it is not uncommon to have fans starting and stopping..

That a nice computer you have there. Based on your description, my guess is you have a bad power supply.
About a $300 fix if you send back to HP. Which is what I had to do a while back.

Check the Time and date settings in BIOS

Try removing the battery from the unit and re-connect.
Do this without the adaptor plugged in.
Next, plug in your a/c adaptor and see if the power light and battery starts to charge.
You may want to try at last resort, after the unit is running without the battery, install you battery and see if the charge light comes up.
Sometimes, just removing the battery and re-installing it cures the problem.
If you do not recv. a charge light and the unit will not start, HP has offered a extended 1 year warranty on top of you the original 1 year warranty.
Contact customer support by email and log your problem.
Hope this info has helped.