I have a faulty sensor at the moment. First thing I'm going to do is check the connection to the dials by removing top half of dash and checking everythings plugged in ok!
If you have an early 172 like mine (2002) or a cup, then I beleive the speedo sensor is located on the top of the gear box, and apparently takes 5 minutes to change. I haven't done mine yet as it's working OK at the moment. This should give you an idea of where the sensor is
http://www.cliosport.net/forum/showthread.php?t=487994&highlight=where+is+speedo+sensor
I'm gonna do mine over the weekend if Renault have any sensors in stock.