I could be wrong here. Help me out.

I recently replaced the single-wire fan control coolant temp sensor in the side of my head with a aftermarket sensor from Hypertec. When I did this the coolant gauge stopped functioning because of, I'm guessing, the way the sensor works. I didn't realize they were tied together. So I switched it back and now it seems I am not getting a signal from that sensor.

I know the sensor is good. But it's not controlling my fan and my gauge still does not function. So I was thinking...

It's a single wire sensor. Therefore it gets it's ground from the engine block right? I used Teflon tape on the sensor because it's the only thing I had to seal the threads. Is Teflon tape an insulator? And possibly isolating my sensor and causing it not to function? Maybe because of no ground?

Yes Teflon tape can cause that. Try tightening the sending unit some more.

Which one did you replace? The gauge sending unit is in the driver's side head, between the #1 & #3 plugs; the fan switch is in the exact same place on the OTHER head, the pass side, between the #6 & #8 plugs. The fan uses a SWITCH, not a SENSOR. (pronounced like Mr. Spock does, "sen-SORE" )

I installed it on the drivers side between 1 & 3. That would explain everything. The gauge not working, the fan not working, etc. I was not aware at all of the switch on the pass side. And thank you for verifying my thoughts on the Teflon. I will correct it tonight.

^^ Ockham's razor, a principle to live by...