To what parts are the temperature sensors connected in my G4 iBook?

The screen on my iBook started freezing after a few minutes or even during rebooting. I found a website that recommended re-soldering a few pins on a certain chip. I had my husband do that, and I installed the G4 Fan Control. I've put the new temperatures at 36, 33 and 37, lower than recommended, but I wanted to see if overheating was the issue (as opposed to the fan not working) The iBook is not freezing as often. However, I've been watching the temperature sensors as I use the computer, and I've noticed that sensor 2 heats up the quickest, causing the fan to come on. If it gets a degree or two above my new setting, that's when the computer freezes. What is sensor 2 telling me, and hence, what part might I need to replace?