This week’s repair is a first-generation unibody MacBook Pro that showed intermittent signs of video distortion in one user account but not in others. Our customer is a graphic designer, so we set her up with a free loaner computer while her Mac was in for repair. (Did you know that Small Dog offers free loaners to customers who buy their computer and AppleCare from us?)
Usually in this situation, where a machine exhibits misbehavior under one user but not the other, software corruption is to blame. This case is different, though, because the first generation MacBook Pros have two graphics cards on their main logic boards. One is more powerful than the other, and you can opt to use one or both to either save battery life or increase performance. This is done in the Energy Saver preference pane in System Preferences.
Our customer had one account for herself, which had the higher performance option selected. The more powerful graphics card had failed, causing the video distortion; when she logged out for her daughter to use the machine under her own account, with the better battery life option selected, there were no signs of trouble. So, we ordered up a main logic board (all Apple laptops have the graphics processor(s) soldered to the logic board), and the trouble went away. It’s not too often that the tried and true method of software/hardware isolation fails us, but a thorough knowledge of the product in question is what saved the day.