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I have a Lenovo ThinkPad T420 with an i5 processor and integrated graphics. I dropped the laptop and now the screen is not working. I replaced the screen and it is still not working. It shows the BIOS and then at Windows login the screen gets all static looking. But if I plug in a DVI-to-HDMI cable the laptop starts and works fine. Even the external display works and the laptop display works too. As soon as I unplug the cable the screen crashes and is all static.

I cannot figure out the problem. The video works because the external screen still works. How can I fix this?

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Use the Thinkvantage Toolbox to test the Video subsystem, see below:

enter image description here

Based on the test and whether you can get the device info, you should be able to see which component that is failing. Since you can use the external screen, part of the Video subsystem is working, so it is more likely that the connection to the Thinkpad Display and/or the new display itself fails.

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Thank you so much i really appreciate your help.Testing it right now The fps in video test are awfully low about 7 frames per second and the video port test came back with a bunch of errors .But i had no external monitor attached yet . i.ll post the error log later . –  marien Nov 21 '12 at 11:50
    
Great, don't forget to vote up my answer or accept it if you find it ok. –  FredrikD Nov 21 '12 at 13:04
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