After some moderate searching, I’ve drawn blanks and would love some insight from the Club 17 crew. I went ahead and cleaned away the original garbage thermal paste and thermal pad on my A1139 to throw on some new, much better paste. [This all went without a hitch, but oh my lordt, the state of what I had to clean out was breathtakingly gross and disintegrated. Pics of the before/after by request.] What prompted this thread question is a step in iFixit’s disassembly guide for reaching the heat sink, specifically Step 28. This was the moment when I noticed how there’s an LED on the logic board and a plastic, dome-shaped cover described as a “lens” which needed to be gently removed on the A1139 to access the Torx screw partly covered by it (this step for earlier PowerBook 17" models is slightly different and doesn’t require dome cover removal). Whatever the case, cue my genuine surprise. I’ve never seen the power button illuminate on this PowerBook under any condition (i.e., powering on/off, putting the system to sleep, etc.). On 15" aluminium models (A1138 and prior), this area is described solely as an ambient light sensor, not an LED. On the 15" models, it doesn’t appear to be a dome. Are there special conditions wherein the 17" power button is designed to illuminate, or was this a latent feature that was left unimplemented? Or, more likely, is it that iFixit’s description of “LED” here is erroneous and merely one of the solid state ambient light sensors?