Why is my 06 PT Cruiser Overheating???

My 06 PT cruiser is overheating.  I changed out the radiator cap & thermostat & the coolant level was
fine, no loss of coolant.  This took me an hour and a half so the engine had cooled significantly, then
when I started the car the Temperature gauge was immediately all the way up to HOT & the car had
not even run for 1 minute.  I took off the radiator cap & looked to see if the coolant was circulating in
case the water pump may have failed, but it seems to be circulating the coolant.  My only other
guess is the temperature sensor may be bad.  When a mechanic changed my timing belt he had
forgotten to plug in the temperature sensor wire, so the car always showed the temp as flat "Cold"  
once I plugged it in it showed it as right in the middle where it usually is.  Could this wire have
shorted out & caused the engine to "think" it was over heating?