Cooling Problem

I have these 2004 sentra with the engine light on plug in the scanner found this code P0128 Coolant Thermostat(coolant temp below thermostat regulating temperature).So what I found out is that if I put the heater on the temperature gauge neddle drops and if I turn off the heater temp gauge neddle goes up again could these means that the thermostat is stuck either closed or open or coolant sensor is bad. Wolud youguys please tell me what could be it and also where is the coolant sensor located at
Thank you

I have these 2004 sentra with the engine light on plug in the scanner found this code P0128 Coolant Thermostat(coolant temp below thermostat regulating temperature).So what I found out is that if I put the heater on the temperature gauge neddle drops and if I turn off the heater temp gauge neddle goes up again could these means that the thermostat is stuck either closed or open or coolant sensor is bad. Wolud youguys please tell me what could be it and also where is the coolant sensor located at
Thank you

I would change the thermostat, sounds like it is stuck open.

Go to this site:

http://www.nicoclub.com/FSM/

Scroll down to Sentra, click on it
select year
go to Section CO then to page 12 (Thermostat location)

Go to page 30 in section CO for temperaturre sensor location (According to picture it is located in the water outlet housing.

Hope this helps