Question: I have a dell latitude 820. The fan was blocked for a while, after removing the crap (hair, etc.) the fan didn't work anymore. I tested the fan, but that part was OK. So I think the fan controller is broken. Which component is the fan controller?

Answer: 
If you are sure the fan is working then there isn't much that you can service. Not sure about your specific laptop but most have the fan controllers built into the motherboard so there isn't much to fix (beyond replacing the motherboard).

Are you sure the fan just isn't turning on because it doesn't need too? The d820 we have here doesn't run the fan very much and it is fine. I did find this utility which claims you can control your fan by software, this may help in troubleshooting if the fan is truly broken or if it just isn't turning on because the temperature is low.

[Dell Fan Control Software](http://www.iterasi.net/openviewer.aspx?sqrlitid=hzoigkl74kqrmccelopr-w)
If you enter the BIOS on most Dell Latitudes you can run a diagnostic on the system and determine what may be wrong. It will test the fan and controller. As others have mentioned, they don't seem to run all that much normally