I recently got a ps3(fat) 80gb model that has ylod, when i boot the console up the light goes green, yellow, then a fast tripple beep then starts blinking red until i shut the console down. I've replaced the thermal paste with arctic silver 5 but the error still exists. Do the error codes above mean the ps3s overheating?

You have cold solder balls under the chips. They need to be reflowed or reballed. Depends how bad the damage is. This is caused from overheating.

could also be a PSU problem.
http://leham-forum.freegametalk.com/t27-ru...-a-xbox-360-psu

Could be, but in all the YLoD PS3's I've seen I think I have only had 1, maybe 2 that ended up being the PSU. YLoD just means general hardware failure and 99% of the time it's as MadMaxGR said, cold or cracked solder joints which will need a reflow, or preferably a reball.