I have a 2003 sebring LX sedan 2.4L. My thermal guage in the dash never goes over the first two notches. The guage itself works since I have done the test to see what all works in the dash.

How hard is it to replace the sensor and is is ok to put off the replacement?

I drive 42 miles at a time so the engine does get up to operating temp.

 

My 2001 and my 2002 both do the same thing. If you are getting good heat out of the heater in the winter, the car is probably warming up fine.

 

more then likely broken thermostat

 

Where is the thermostat located and how hard is it to replace?

Are there any ill effects from putting off the repair?

 

fallow the heater hoses

 

The thermostat housing is right below the radiator cap on the 2.4L engine.
"Ill effects of putting off the repair": Slightly worse gas mileage, cold toes waiting for the engine to heat up.
Once you get the thermostat out you can test it in a pot of hot water on the stove.