Apple has used both light emitting diode and liquid crystal display screens in its MacBook Pro line of laptop computers. While the technologies behind these displays are different, both may experience an error that causes horizontal lines to appear on the screen. This problem can have multiple causes, making it difficult to diagnose and fix without the help of an Apple-certified technician.
Whether you have an LED- or LCD-powered screen, the display connects to the video card inside the laptop's chassis. Binary data travels from the computer's CPU to the video card, which then translates the data into images that appear on the computer's display. If the components or the connection between them is faulty, a number of errors may occur, including your MacBook Pro having horizontal lines on screen.
If your MacBook Pro has blacked-out, grayed-out, flickering or faded horizontal lines going across its screen, the problem most likely is due to a poor connection between the laptop's display and its internal hardware. The only permanent fix for such an issue is service by an Apple-certified technician.
MacBook Pro Lines on Screen Fix
A quick, temporary fix for a poor connection between the display and internal hardware is to reposition the MacBook's screen. If closing the laptop and reopening it doesn't work, you may be able to adjust the angle of the display until the horizontal lines no longer appear. If adjusting the position of the computer's screen doesn't result in any changes, the problem might not be hardware-related.
Faulty Video Card
While this is less likely to occur, a faulty video card also could cause your MacBook Pro display problems. Apple reported in July 2008 that some of its 15- and 17-inch MacBook Pro models containing the NVIDIA GeForce 8600M GT graphics processors had faulty video cards. The remedy is a new video card, but this issue cannot be diagnosed or fixed without opening up your computer's chassis – an action that should only be performed by Apple-certified technician. According to Apple, there's no charge for such repairs "if the NVIDIA graphics processor in your MacBook Pro has failed, or fails within four years of the original date of purchase."