Okay here it goes guys:
First let me note that I have one of the first PS2 models released, it is around 5 years old . I bought GT4 the week it was released. My copy worked perfectly, I left it on for almost a week. Turned it off one night and the next day I couldn't get it to play to save it's life. So I took it back to the store, got another brand new copy, still didn't work. Since my box is already cracked open ...
(I spent a lot of time realigning the laser to read my blue disks before I learned from someone in the video store to PUT ABOUT 1" OF SCOTCH TAPE ON EITHER SIDE OF THE HOLE TO PLAY BLUE DISKS if your machine is giving you a disk read error. Has something to do with the weight of the disk.. blah blah. None of my blue disks would play, now with a little tape, all 4 of them play without a problem) ...
I noticed while it was trying to read the disk that it wasn't spinning fast enough, so I got out my handy can of air. This I pointed at the edge of the plastic center piece and when the disk start spinning, I gave a good shot of the air... this worked... until i ran out of air.

So the game got put on the shelf for a couple of months.
So recently for christmas I was given "God of War" ( 🤬 awesome game) and when I first tried to play it, it wouldn't read. Well I was just not having this so I uncovered my PS2 and began realigning the laser (adjusting the white cog in the back of the disk drive) after going around a complete 3 times and having a couple hours into it, nothing. So I hopped on the trusty internet. This time around I found a few articles about recalibrating the laser instead of just realigning it. This seemed to fix a lot of disk read errors, or so claims stated. There are many hypothesises on why the PS2 laser needs recalibrating, a lot of them include watching dvd's in your maching (something about the laser constantly reading during a dvd, whereas it loads and stores game information in the ps2). Well this repair is very expensive $80 - $120.
Well obviously my machine was worthless after learning that, because a used machine around here runs about $100. Knowing that I decided to try this recalibrating thing myself. I took my PS2 completely apart (laid out a big towel on the floor). I had to take it completely apart to unattach the disk drive (possibly because mine is quite old). Disk drive upside in hand, I then had the underside of the laser (very small motherboard surrounded in the plastic casing around the laser) facing up towards me and I noticed 2 small very bright silver screws. I turned them both 1/4 turn to the left and I half-reassembled my machine. Put in God of War, the disk wouldn't even spin. I then put in San Andreas (which has always worked), wouldn't even spin. I thought then that one of the cords to the disk drive was not properly attached, so I took it down again and reassembled making sure to plug in every cord properly and snugly. Disks still would not spin. I took my machine completely apart again and with the disk drive upside down in my hand, I turned those same 2 silver screws on the motherboard 1/2 turn to the right this time, so 1/4 right of starting point. I half assembled my machine once more, GTA played! So then I put in God of War, and it played! 2 succesful resets with God of War (the game that never worked even once)! Very excited at this point, I crossed my fingers and put in GT4, and it worked! I put my machine back together and voila.
It was just a matter of recalibrating my laser.
P.s. Don't go tearing into your machine, then get pissed at me when if you can't get it back together

. I'm not recommending that everyone go and do this themselves... It was a tedious, time consuming repair that requires a steady hand, a very small screwdriver, and an hour of silence. However it was not as nearly as difficult as I had initiatly anticipated, definatly not worth $100 in my eyes ( having done it once, i could do it again in less than an hour).