thermostat (p0128)

i have a 2000 sunfire, 2.2l. the other day i noticed that the car was not heating up to proper temp. it sits between 40 and 90. i got a code that is p0128 (coolant thermostat - coolant temperature below thermostat regulating temperature ). is that the thermostat or one of the sensors that is miss reading,. if its the thermostat where is it located. and on the rad i know where the drain valve is but can not turn it to open it , wanter to drain and add new coolant. is there a trick to opening it.
thanks
tommy