The speedo on my 172 phase 1, also stopped working.
I bought a new g/box sensor on Ebay for £5.... and it worked OK for 3 days. It then went intermittent. It usually works OK when the engine is cold though, but fails after a few miles.
I bought a second new sensor on Ebay. It also worked fine for a few days... but has now gone intermittent.
Is there a problem with the Ebay sourced sensors ??? Any ideas ?
Incidentally if you brake hard, and trigger ABS while the speedo isnt working, then you get the ABS lamp illuminated on dash.
The lamp clears again by itself after the speedo is fixed.