Temp gauge mystery

When I start my slant6 engine and warm the motor the temp gauge needle doesn't budge. Thereafter and everytime I turn it off and then back on, the needle sweeps nearly all the way to the right(somewhat quickly) then sweeps back to the left and settles at zero, and stays there. I tested the wiring circuit last fall and it checked out okay. Has anyone a clue as to what's going on here? Thanks.