Year of vehicle - 2007
Make of vehicle - Pontiac
Model of vehicle - G6 Sedan
Engine size - 3.5L V6
Mileage - 55k

P0128 - Coolant Thermostat (Coolant Temp Below Thermostat Temp)

Check engine light came on a few days ago and I noticed my Engine Temp Gauge was not working. The needle is always burried at the bottom and does not move at all. My heat does work so I'm thinking the Coolant release isnt stuck open. Stoped by Autozone and they said its probably my sensor. What sensor are they referring too? Coolant Temp Sensor? What else should I check out before buying all kinds of replacement parts? I cant afford to do that anyway. Thanks all.