My ford explorer temp gauge stopped working properly after new radiator was installed

My ford explorer was alternating between normal and almost to hot every
few minutes . When I bought it to the shop, dumb me did not know it was low
on coolant cause it did not leak and the overflow wasn't losing much water.
the gauge WAS WORKING when I brought it in . the guy put in a new
radiator told me the old one had a leak on the top seam.  $400 later, the
gauge for the temperature immediately on my dash now runs cold, barely
getting up to the C on the gauge.   car runs fine but he said oh well now you
need a new gauge. the guy put it on the machine and said everything was
working. Well how can that be right? the gauge does move slightly as it
heats up but stays @ C  I grounded the temp sensor  and turned it on but it
didn't go to H ...heater works too... I also got all the air from the water
system  I drive the car short distances ,but check the fluid each time before
getting into car which is becoming a real pain. would a sensor go bad that
quick? it never got real hot.... Any ideas?  Thanks in advance! Anna