Water Coolant Sensor

I've had a check engine light on for awhile. Autozone discovered code said coolant temp was an issue. Truck was not warming up quickly at all in winter. I installed new thermostat which quickly got the engine warmer quicker. Still have check engine light and Nevada will not register vehicles with check engine lights on.

It's been a tough search looking for pics/info on changing the coolant sensor. I've read where my truck only has the head sender (single wire) for the gauge. I've also read where both the sender and the sensor are present.......somewhere, lol. I understand the sender is for only the gauge and the sensor is for the PCM. Will both cause a check engine light? My gauge has been mostly far left (cold) but responds occasionally.

Can anyone steer me in the right direction? Thanks for any help.