lap top turns on and shuts off after 3 seconds.

[Harry_70]

I have a M6300 Dell. An old laptop that was working fine. Yesterday I decided to to have a routine service.I dissembled it in order to replace the thermal paste on both CPU and GPU processors. After I cleaned the fans,cleaned the surface of both chips, I put a good thermal paste and put it back.
Everything I did was according the book and nothing less or more.When I turned it back on,the power light stayed on for two to three seconds and then it turned off. I went through the procedure of the static electricity etc... Nothing helped, or the re seating of the Ram. What causes a perfectly working laptop to behave like this. Any wizard who knows a bit more about this. I would appreciate any serious help.
Thank you

 

[Ironsounds]

Hello... re-check your CPU fan connection to the MB... if there is no Tach feedback from the fan, then there will be a Boot stop.

 

[Harry_70]

Hello... re-check your CPU fan connection to the MB... if there is no Tach feedback from the fan, then there will be a Boot stop.

Thank you friend, I tested the fan and it looks fine. When I turn it on pressing the fn button, the number 9 light its flashing on which is the first of the group of 5 all next to power button.
Need to find a troubleshooting to see what the flashing means.