回复： Mac display screen works only when external monitor is connectedService people who work on replacing the GPU on MBP 17 motherboards says that if the graphics look OK on an external monitor then it's not the GPU. So they are reluctant to accept taking on to replace the GPU on my machine. If I disconnect the external monitor and use the gfxCardStatus app to switch to "Integrated" (Intel graphics) then the LCD display perfectly. But the only way to connect an external monitor (and I need that) is that the "Discrete" graphics (NVidia) is active. And then as mentioned the LCD does not show colors correctly although the external monitor works. What should I troubleshoot ? Is this a known issue with solutions ? The only case I see out there where people are having trouble is where the *external* monitor is acting up.