This is the easiest thing you can do to attempt to fix your PS2, while it hardly ever works it only take a minute or two, and should be done first before you take apart your machine. 1) You must be in the "main menu" screen of the PS2 console, the first screen that comes up when you start with no disc in.
2) press the "triangle button"
3) Select "Console" from the top using the up directional buttons and press the "Triangle" button
4) Select "On" using the up and down directional buttons and press the "X" button 5) Insert a disc. The console automatically starts the diagnosis.