a real answer. not bs above.
you proved the PSU is ok, why go there?
the why this works is the chip called South bridge.
does all this (this is how (also) the windows start button and shut off works, called SOFT POWER OFF)
the button on the front panel is a N.O SPST switch.
normally open momentary and shorts a specific and non world standard pin on that mobo to ground, (adjacent pin)
this sends a logic zero state (0v) to the South bridge
then it turns on a transistor on that mobo and that grounds pin 16 to PSU, and PSU magcially turns on.,
the problem can be ; the switch is plugged in wrong. or bad.
HP uses vast numbers of MOBO in PCs so have all different pinouts. and hard super hard to trace and learn that.!
all mine are keyed, and the mobo is silk-screened painted, Front panel.
the other possible is that the PSU is not putting out standby power
on pin 9
, with PSU plugged into the wall pin 9 must be at 5vdc.
if not the South Bridge will be dead.
as will the power switch it supports
The South bridge runs power. managment.
the 945P must have PSU standby power and the switch most reach its power on pin, or there will be no working switch ever.
The PC can also force the PSU off if one of the onboard
power regulators are reading wrong (say too high)
The power management is complex, but not that one pin,.below.
wake on LAN, sleep, hibernate. and more.
but all we care is the power switch but if the above features work then the switch is bad, and why I mention that.
my PC can even self turn on at 6am if I want, (a bios features) self power on...
now the list of simple..tests,etc.
1: PSU pin 9 must be 5vdc at all times, wall plug aC connected.
and if read of PSU has a switch, set to logic 1 , (1/0 labled)
2: the correct working switch in front panel,
2b< that switch must be on the correct jack on the mobo and not off one pin or some other ERROR. there.
3:the PSU good ... some are not, (complex reasons) skip i will.
4: the MOBO most be good, (and not overheated)
will the PSU turn on if dvd/cd drive and HDD drive cables pulled.?
even pull RAM. sticks? it will beep like mad but will turn on.
and be dead but turns on. like that. well most do.
5: jump the pin below, does that turn on the PC? as it must!
A DT PC 12 years old now.
this PC is now retired at HP means zero support.
no service book but it lacked the power jack info in most cases
DATA: (first know what is there, the mobo is clearly labeled. look?
this is the best search I did for you,at HP real, yours
- Motherboard Name: P5LP-LE
- HP/Compaq motherboard name: Emery-UL8E
ive owned many P5 in my life, not now but am an ASUS guy.
but this old board we can do research on it,
I like HP and HP likes ASUS as do I. both.
knowing this fact
Asus Leonite P5LP-LE User Manual
then we find the book on it next like this, (took me 1second )
page 5 shows layout
bottom right shows, (F_panel)
Bingo, on jack.
page 25 covers that jack
so pull the jack there pull plug from jack
then jump pin gnd /pwr pins with screw driver tip.
if PC turns on the switch was bad or plugged in wrong,
if not the PSU is dead, Standby power is dead, pin 9
Asus books always show all this, but never HP for some odd reason. (details like this)