i will give you my definitive guide on things that are crappy at reading optical media as they all use generally the same parts. ive gone through about 20 original playstations where the only problem was crud on the lens.
toploading things are easy to check, just open the lid and stick your eyeball in there. put a little isopropyl alcohol on a qtip and wipe it off gently, dry it off with a can o' air and see what you get. the infernal frontloaders require a complete disassembly :< though generally what you're looking for when you take it apart is a lid on top of the drive. they have a little doowidget spinny bob on top that usually just has like 6 or 8 tiny screws. but not always it depends how much they hate you. when dirt and crud gets sucked in there its eternal, every fat ps2 thats passed thru my hands was filthy inside. if you take your time you should be able to get to the lens eventually and will almost certainly find it coated in dust or the dreaded smoke heavy household yellowish film of despair.
90% of the time that's all you'll need to do. you'll feel silly you didn't just do it years ago :O but sometimes it is actual weakening of the laser. everything with a laser has a little potentiometer somewhere to limit the voltage going to it, most cd based consoles only have one, but the original ps1 model had 4 of them for some reason :I later models didn't maybe they decided it was dumb. dvd based things always have at least 2, except the gamecube cuz it was never meant to read cds. anyways :o
don't even bother trying this without a multimeter with an ohm setting. internet yahoos will tell you just to crank away at those things with no regard but that is terrifying :O by increasing voltage to the laser you can potentially increase it's ability to read discs, but doing so can also decrease the lifespan of your laser diode. or just make it pop if you go nuts in there, so beware.
the only challenge here is actually finding the stupid things, they can be on a little pcb up under the laser assembly, stuck on the laser assembly itself, attached to an adjacent board, or just way out in the middle of nowhere. keep your eyes open when you're taking stuff apart for those little thingers.
this is someone elses picture of the original xbox potentiometers, they're on a ribbon attached to the laser assembly. as you can see they look like little phillips head screws, but it's hard to describe how tiny they are. once you've located them you'll want to put your meter on 2k ohms and put a lead on both legs, i circled legs in blue :o if the legs are too impossibly small to get 2 leads on at once then put one on the screw and try the legs individually, if it goes to 0 or OL then try the other leg, there isnt any universal standard for which leg is meant to be the output you'll just have to find out on your own :< if you don't get anything then change your meter to 20k ohms and try again. from system to system and even comparing identical systems the number you get can vary wildly. gamecubes are typically 200-400ohms, xbox360 can be like 4000-6000ohms. just make a note of what the factory setting is.
once you've worked yourself up into a frenzy take a microscopic screwdriver and make a turn so impossibly small that you can barely notice youve moved it and take another reading. the direction you'll want to turn the potentiometer is also not standard, so you'll want to know which way is which. here is your super goal, lower the resistance at the potentiometer to allow more juice to go to the laser, but no lower than what you minimally require. if the original setting is in the hundreds i'll lower it in stages of ~50, if its in the thousands i'll drop it by ~100s. this is boring and time consuming, but have a disc on hand and reassemble it just enough to see if you're getting any improvement.
the dangerously low setting is also different between consoles, if i dont see anything helpful after getting as low as 3/4 of the original setting then i may have to conclude it just isn't going to help. sometimes the problem is different, like a drive motor getting stuck. and setting the resistance too low can also keep it from reading discs correctly.
almost everything i've run across will give up and start working after this process, and if not you can at least conclude it's not the lasers fault. unless it's not reading any disc at all ever.
clean the lens first. it's almost always the lens.