Check engine light on, code p0128, where is the temperature sensor located?

P0128 is usually not the sensor but a bad thermostat that is stuck open. It causes the engine to run cold and not warm up properly and you get the code. However, the sensor is on the firewall side of the engine screwed into the coolant block next to the heater outlet tube below the exhaust manifold about in line with the O2 sensor.

I do still get heat in the car, and it does feel still like A/C as well working. We had cleared the code, and the gauge worked just fine, for a few driving miles and then CEL came back on, and the gauge stopped. This is why I am leaning toward it being the sensor, and not the thermostat....

Okay, in that case check the wiring harness, if the gauge worked okay, it's likely the wiring harness, either corrosion which causes high resistance or a bad connection doing the same. That system works by measuring the resistance in the sensor, the lower the resistance, the higher it thinks the temperature is. So, if there is a bad connection or corrosion that raises the resistance, and the ECM thinks the engine is staying cold and sets the code.