Fn+z is indeed a hidden Dell bios function. Why Dell chose to keep it a secret is beyond me. It not only resets the temp sensor reading, but also successive key presses manually cycle the fan speeds. If you have the time, here's an interesting story of how I just recently discovered this hidden bios "jewel" myself and got a free laptop out of the deal.
A good friend had a 9200 that had just recently overheated from excessive dust in the CPU and GPU heatsinks and was running downclocked at 600Mhz instead of nominal 2Ghz. Even after he disassembled and thoroughly cleaned it, it would only run at 600Mhz no matter what he tried. To add to the frustration he figured the bios was corrupted and was going to reinstall it but it wouldn't allow it until he replaced the old weak battery. Because the laptop was in great condition he bought a new battery then reinstalled the bios. When that didn't fix it he bought another CPU thinking that maybe the CPU had been damaged during the overheat. Once again, no joy. He finally threw up his hands in sheer frustration thinking the motherboard was bad and just gave me the 9200 thinking I could use some of the parts as spares for my M1710.
After receiving it the first thing I did was to boot it to check the bios. It confirmed the low operating speed, but unfortunately the 9200 bios doesn't display temps so had no clue as to what the problem was. Both fans were running at max, even when just booting from a cold condition. I then booted into the Windows 7 RC he had installed and noticed he had already installed I8kfanGUI, which was my next step anyway. It didn't matter if the computer had just booted or had been running for awhile the CPU temp was always at 87 degrees C, which meant something was wrong with either the temp sensor or the temp reporting system.
Next, I disassembled the laptop and checked the fans and proper CPU heatsink contact. I even looked for hot spots or anything out of the ordinary on the motherboard with a magnifying glass. Everything looked fine. He had also done a proper job cleaning. I went ahead and reinstalled the CPU heatsink with Arctic Silver 5 then reassembled it. Still, no change and fans still running at max all the time. Of course, the 9200 was running dog slow.
Immediately after that I searched the forums here. Surprisingly, I didn't find anything. After a couple of weeks of occasional Googling different ideas that would pop up I finally found a forum where someone complained that her Dell laptop fans were running wide open all the time. Way at the bottom of the forum page someone mentioned the Fn+z function. WOW...gotta try this, I thought! Just as soon as I hit the keypress the fans went to idle and I8kfanGUI immediately reported CPU speed at 2Ghz and 43 degrees C. Windows was suddenly snappy. After rebooting a few times and running it hard with no problems I replaced the Windows 7 RC he had on there with the final RTM and the little 9200 has been running perfectly since. My girlfriend is now teaching her young kids on it.
When I told my buddy that the 9200 was repaired with a simple keypress he uttered a few expletives, but at least was laughing about it. He didn't want it back since he had already bought a replacement computer. I guess I should be happy that he hadn't read my "How to" sticky here on fixing heat related problems with these older Dell laptops, especially the importance of keeping the fans/heatsinks clean to avoid overheats!