I have two (60GB) CECHC models of which one is a bit newer. Both did have YLOD and I have had one permanently fixed. Using lead solder, which happens to be safer than non-leaded solder (do your research, RoHS never took into account other factors than lead). I do not need to have the YLOD fixed on the other PS3 just yet unless it gets worse or if I get tired of it because it only happens when I either restart or start back up when its been warm, kind of weird, so could just be a whiskers (NASA's own wording) of the non-lead solder creating a short, but not so sure really.
The PS3 that has had its YLOD repair is the one that has a fan as loud as a jet engine, it actually sounds like a jet engine does when it is warming up at the airport ready to go. When I turn it on it is silent but after a few seconds if I use XMB menu it will start to increase slightly audible then when I load a game or app it will go louder again until it is in full throttle jet engine mode when I start using some power of the components. I see the PSU as the main output of heat.
Not sure what you guys mean when you said it is passive cooling for PSU? Unless we use a fanless and or water cooled one with heatsink it is not passive. I see PC fanless PSU referred to as being passive cooling.
I bought an external cooler for the silent PS3 I have that suffers from YLOD, I have it plugged into its own USB power source so when I switch it off it still cools for as long as I need and while powered on it cools slightly better, but not so good because of the poor design of the PS3 PSU. I know it works well because it will allow me to boot up my PS3 again right after a gaming session if I want, so I just let it run for 1-2 mins then start PS3 again if I need to. Also I feel the heat coming out faster so it works well but it isn't going to cool it down a great deal more. I just see noisy fans as a fault as my silent one doesn't get any hotter than my noisy fixed PS3 does. They are different though because I measured their power use and as I remember the PS3 with silent fan uses about 30W more when idle. Say my other one uses 150W then the other uses 170W. Not sure why, maybe its just because it is an older model and not as energy efficient.
I also have a broken PS3 and taken that apart and it has dust, but again it is YLOD and needs fixing or selling as is. I think it is to do more with the model its date of manufacture and the number of blades on the fan used. We know the larger the diameter of a fan the quieter it is. My 120mm fan in my PC is silent, a reliable fan should last at least 30,000 hours.
Now back to my other two PS3s.
The one that has been fixed for YLOD was brand new refurbished Sony repair. The actual case is brand new and everything is as far as I know. Not sure what they replace or fix but I thought maybe Sony thought it wise to make fan rev up at full speed at lower temps, so I was thinking since Sony have had their hands on it they did this deliberately thinking it may prevent a YLOD which it never will. Every PS3 and most electronics will always fail because of non-leaded solder. XBOX360 has about a 66% fail rate whereas PS3 is only 2% or something like. This is still a lot though. Maybe the PS3 fail rates are out of date, I don't know.
I am really not sure what to do. I will see if a replacement fan will fix the noisy PS3 and if not then it has to be a fault with the sensor as you were saying, but I doubt this because it being a Sony refurb they would not send a faulty repair out.
I've seen some fit water cooling to their PS3 or just to use external PSU. And I think everything should have the option of allowing external PSU anyway, especially a PC.
I have one of the original 1st gen fat's (the 60GB model which essentially had everything), and have yet to run into a problem. It may be that I am lucky, or more likely that I keep it clean. I replaced my thermal paste about a year ago (3-4 years and most pastes are done). Every 4-6 months I take it apart and clean it out, checking that the fan can still spin freely without ball-bearing noise.
You need to think of it more in terms of a PC than a console... If the fan is spinning up that high, there is probably a reason. You either have poor air flow, bad contact on the heatsinks, or a failing thermal sensor.