Although the Playstation 2 is a system well-known for it's longevity and durability, the fact that it is over a decade old means that it inevitably will have a few issues. The most common of which is the infamous disc read error. This is characterized by entering a disc into the PS2, but instead of booting in the game, an error message comes up saying the disc could not be read or the disc was not recognized. There are several ways to fix it, listed below in order of least to most drastic. This article will be focus on the SCPH-30001 model as this is the model I have personally worked on. The 8 screw version follows the same guideline, although is notoriously harder to get to the laser.
Cleaning Your Disc
Of course, a disc read error may not even be the fault of your PS2 if your disc is unreasonably dirty. There are several ways to clean your disc, but the easiest and most effective of which is to get a disc doctor. You can also take it to a local pre-played video game store and often they will fix the disc for merely a few dollars.
Cleaning Your Lens
If cleaning your disc doesn't work, try cleaning the lens. This is commonly the problem and is extremely quick and easy to do. Just take a q-tip, dip one end in rubbing alcohol, and apply it to the lens carefully (not applying too much pressure). Allow the rubbing alcohol to dry and give it a try.
Adjusting the Laser Voltage
If the prior two steps don't work and you are confident that the steps were done correctly, you can try this last trick. Please note that this WILL void your warranty. This probably isn't a problem since the PS2 is so old that few have existing warranties. You will need a jeweler's phillips screwdriver and a normal phillips screwdriver.
First, flip the ps2 on its top, so the bottom is facing up. Pop off all of the small screw covers with a flathead screwdriver, then unscrew all of the bolts. Next, take a knife or something sharp and cut the warranty seal off, or slit it along the seams. You need it off so you can take off the bottom. Now, take off the bottom of the ps2. Make sure to lift it directly up and off, as the case can snag on a cable.
There are two screws that hold a metal cover and the expansion bay down. Remove the screws and the metal cover and expansion bay. The expansion bay cannot be removed entirely because of another ribbon wire. Now, the fun part. While you're holding the expansion bay, you will see two small silver screws on a ribbon, on the drive. These are what you will be adjusting. One screw is the dvd screw, one is the cd screw. If you are facing the expansion bay, the left screw is the DVD screw. Adjust it slightly clockwise, very very slightly (about 2mm clockwise). Put the expansion bay together (not screwing anything, just in case you have to readjust), set it inside of the plastic case, flip it over, turn it on, and test a disc. If it doesn't work, adjust it a tiny more. If you go more than a quarter turn and it is still not working, then you need your laser replaced, as it has been completely worn. 
And that's it! Hopefully this will bring your PS2 back to life, and you'll be playing your favorite ps2 games again. If you have a newer 8 screw model on the bottom of the PS2, its basically the same tutorial, but its a bit harder to get to the laser. I don't exactly know how much harder, I have never worked on a non 10 screw ps2. If you're not afraid to open the console and break the seal, you can just open so the top comes off, unscrew the tiny phillips bolts that holds the cd tray down, and access it that way also.
