Before we answer any questions, you need to know the basics of how an optical disc drive works.
A CD spins on the spindle, and the laser reads, moving outwards from the middle of the disc. Think of the data on the disc as a spiral, just like vinyl records, from the inside to the outside. The laser shoots out of the lens, is reflected by the disc, then goes back into the lens and is redirected to a photodiode that responds to some light (CD: infrared light, DVD: red light).
Data is recorded as “pit"s (actual potholes in the media) and “land"s (no depression). The depth of the pit on the CD <0.2 micrometers, so that the light reflected from a pit will cancel the light reflected from a land, so that the difference is very obvious to the photodiode (source).
Now, since you can’t even see the spiral track with your bare eyes, it’s unrealistic to expect that the simple worm gear (coloured arrow) is precise enough to position the laser exactly where it needs to be. That’s why the lens can move sideways (coloured arrow) as well - this is called the “tracking”.
We’re talking about light here so of course we also have to deal with focusing the laser. And since you can’t expect every disc to be perfectly flat or not wobble while spinning, the lens also has to be able to move up and down to always keep the disc in focus as it spins. And of course there is the electronic circuitry to ensure all this happens automatically.
When you insert a CD/DVD into the drive, the lens will light up and try to find a disc, and try to focus. The lens assembly has two laser diodes built in: CDs use infrared, so you can’t see it, but DVDs are a deep red. The lens has a maximum range of movement, and if it goes too far upwards and it couldn’t find focus, it will go to the very bottom and try again.
This Youtube video (really long) explains it very well.
Now, we can answer questions like:
My PS2 is making a clicking sound The laser couldn’t find a focus, and once it went as far up as it physically could, it slammed down to the bottom to try and find a focus there, just like the autofocus on a camera.
My PS2 is making a rattling sound My fat PS2 made a rattling sound when the worm gear couldn’t move the laser head and the gear teeth were chattering against each other. But to be sure, open it up and see for yourself.
My PS2 won’t detect CDs/DVDs If your laser is old and can’t shoot out enough light, then the photodiode can’t tell the difference between “in focus” and “out of focus”. Makes sense right?
My PS2 can read CDs but can’t read DVDs? As mentioned before, there are two laser diodes, one for CD and one for DVD. If the DVD one is more worn out…
Why won’t it read my CD-R/DVD-R - or reads it slowly Recordable CDs/DVDs are a totally different animal from official pressed discs. They reflect less, and if you burnt the CD at high speeds, the pits won’t be as deep, so the laser has to shine brighter to find something - and if it shines too brightly, the photodiode might not be able to tell the difference between a 0 and a 1. And if it isn’t sure, it will try reading the same part again…
Some SCPH-5000x PS2s (with a V9/V10 motherboard) had a problem where if the laser couldn’t find anything, the PS2 could drive too much power to the laser, burning it out prematurely. Keep that in mind.
What a healthy drive looks like
If all else fails, disassemble the PS2 and open the drive to see what’s going on. Here’s what a working mechanism should look like for a SCPH-50000 SCPH-70000
Replacing the laser
Aliexpress or eBay. PS2 Slim lasers are easier to find than PS2 Fat lasers, which probably means that PS2 Slim lasers aren’t going to be around forever either. Fat: KHS-400B/C Slim: PVR-802W