HP laptop turns on for 3 seconds and then off

i have a HP Pavilion g6-2236TX,worked fine untill it would start to turn off and on by its own some day,that progressed untill the poin that it no longer posts,black screen and only stays on for a few seconds,i have tried a different power supply but nothing changed,i also have noticed that when i plug in the charger (btw i have no battery installed) it has a pattern that i dont remember it having before,the LED next to the power plug thing,when a charger is pluged in,it goes like 2 long 2 short blips of light,almost like a code,i also tried to check the ram,but seemed unable to fit it at the first slot..so the second one is my only choice...tried 3 different ram sticks,all of them i knew worked...so its not the ram i suppose...cpu and gpu are fine,no visible burn marks on mother board...no peripherals pluged,removed the bios battery so it would reset the bios...no luck again..for those 3 short seconds everything including the hard drive works as always,but then it turn its self off...fan is fine,i even tried to just unplug everything and only left the ram and the power cable pluged...same thing...3 seconds on and then off...could this be the end of this motherboard? this laptop is messing with my head and im having the erge to learn to read motherboard schematics just to repair this laptop and live a happy life,please throw in whatever ideas you have.

 

Welcome to the laptop forums site

The important thing is, to check if the fan spins, IF NOT IT WILL NOT START
The cpu heats up in seconds.

It could be a faulty hard drive, try it without a hard drive and see if you get a flashing > icon on
screen.

If you can get into the bios settings then this would eliminate it being a motherboard fault

The motherboard process is checking ram, then the hard drive

 

the fan spins and lives as happy as a fan could,i have changed the old hard drive with one i know works,so it is not a fauly drive and i can not get into the bios as the laptop no longer posts...because it can stay on for 3 seconds and then it shuts off,before this the laptop would work fine and would turn off by its self after some minutes of use,but now it turns off 3 seconds.It sounds like an overheating problem but i really dont think so,the fan is working and nothing seems out of place.

 

You could try removing the cpu,cleaning off the thermal compound, and reapplying new paste.