I have a T400 with switchable graphics and the WXGA+ LED Backlit screen (2764-CTO).  Like many others, my screen failed some time ago, showing a very faint Thinkpad Splash on startup then going black.  Also like others, all of the indicator lights except for the WiFi light are out, and the computer still works fine with an external monitor.  After reading the threads of many others with the same problems, I followed the advice given and first replaced the LED cable with a brand new one (did not solve the issue).  Since that didn't work, replaced the system board with a brand new one, assuming blown fuses as had been disscussed before, which also did not solve the problem.  It seems like by process of elimination the only thing left is the Panel itself?  Or perhaps the LED Sub Card?  This is where I am hoping for advice.

I looked up the FRU's for replacement panels, and it looks as if there are two listed with the same descruption (42T0498, 42T0504).  Does anyone know if there is a difference between these two?  I also read that the LG panels are much better than the Samsung--is there a way to make sure I order an LG panel? 

For the LED Sub Card there are 4 different FRU's (42W7968 , 42W8017 , 42W8231 , 42W8229) so I am quite unclear what to look for there.  Honestly I'm not even sure what the LED Sub Card does.

Any advice is greatly appreciated.

Usually, when the backlight doesn't come on AND the status LEDs don't come on, the problem is blown fuses on the motherboard.  The status LEDs are on the sub card, but they don't relate to the backlight.  The backlight is also controlled by a Hall sensor inside the LCD frame (on the hinge) that is tripped by a magnet in the palmrest.  That circuit turns off the backlight when you close the lid, but it doesn't affect the status LEDs.

The T400 was the first of the T machines to have LED backlights and the fuses were quick to blow.  The panel is the last piece left.  If you replaced the motherboard, there is another unattractive possibility.  The problem could be blown motherboard fuses caused by a shorted backlight circuit in the panel.  If that is true, you would have blown the fuses on the new board when you connected it to the bad panel.

The 2 part numbers are 2 different manufacturers.  If I remember correctly, they were LG and Samsung, but I'm not sure.  Either should work, but if you care, confirm with the supplier.  Bee careful.  The T400 used panels that were different from those on later models.  It is safest if you can find a used whole assembly.

Thank you for the reply!  When the fuses blow, is there any sort of "event" that would indicate they had blown? (pop and a fizz, smoke, lights go on the off, etc...)  When I powered up after installing the new motherboard the indicator lights never came on at all (by which I mean there was no event where I saw them then the fuses blew then they were off).  Thank you also for the detailed explanation on the panels, after som further research I have leanred that the 42T0498 FRU is the LG one.

If I may ask, what does the LED Sub Board do and could it be causing any problems in this arrangement?

Also--not sure if it helps isolate what my problem may be, but when I turn the thinkpad on I cant faintly see the thinkpad flash screen (helps if I have a flashlight shining on it), then I see the windows loading icon, then, as it gets close to loading windows, the screen gets blotchy with light lines running across it before the whole display dissapears.  It is too dark an impage to capture unfortunately.

That sounds like a fuse or a bad panel.  No pop.  No fizzle.  Very quick.  Just a few milliseconds. The only way to be sure is an ohm meter. 

Just installed a (tested) replacement LED Panel and it didn't fix the issue.  Does this mean I likely blew the fuses with the old panel?  I have now done cable, motherboard, and a panel with no change at each step.  Would the blown fuse have damaged this panel or should I be good to try another motherboard with the new panel?

a blown fuse will not hurt the motherboard, but a bad panel can blow a  fuse

So in theory, the panel I just installed should still be fine, and if I get another motherboard (replacing/repairing fuses on the MB is beyond my skillset) with the new panel and new cable everything should work?  Is there anything else it could be that I should target?

I cannot be sure without seeing it, but I think you have covered everything.

Just received and installed a replacement (tested) motherboard, but no change.  At this point I have installed a brand new OEM LED cable, a new (removed from new machine and tested) OEM motherboard, a brand new OEM LG LED Screen, and after being concerned I shorted out fuses on the motherboard with my previous screen, another new (removed from new machine and tested) OEM motherboard.  There has been no change at all in the behavior of the machine.  Still incredibly dark screen, no indicator lights except for WiFi, works fine on external monitor but system doesn't recognize internal screen.  I am at a loss.  From eveything I have read and from what you have counselled, I have covered all the bases. What else could it be? Is there anything else to even check?  At this point I have sunk about $115 and lots of my time into trying to fix this otherwise perfect machine, and am quite frustrated.