MacBook Pro doesn't sleep when lid closes I have a MacBook Pro (13-inch, 2009)

I was changing the Thermal paste which involved removing the board and do some dust cleaning. After reassembly, the MacBook doesn't sleep when lid closes.

What the hardware part(s) that are responsible for this problem?  

NOTE: 
I tried the following:


Restored defaults in Energy Saver.
Reset SMC (multiple times)
Reset NVRAM


I still have the same problem. I am sure its a hardware problem from reassembly I want to know the hardware part to check it
Seeing that you disassembled things to clean everything out, there is a good chance that you forgot to attach (or possibly damaged) the magnetic sleep sensor cable/battery indicator cable assembly.

You can diagnose this by putting a magnet on the right side of the MacBook (where the battery lights are).  If it doesn't go to sleep, it's probably not connected.  Another good test is to run Apple Hardware Test (AHT).  Hold the D key while booting from a powered off state with the AC adapter connected.  If there is an issue with your sensor, it should show up in this test.

You will find the connector to the right of the battery terminal.  If it is in fact damaged, a replacement is quite inexpensive (Apple PN# 922-9061)