P0118 after head gasket replacement and full engine coolant change

After changing the head gasket and changing all the engine coolant in the process, now I'm stuck with a P0118 "Engine Coolant Temperature Sensor 1 Circuit High". The sensor is plugged in of course but it always immediately returns when clearing codes. I sprayed some brake cleaner into its connectors and they look fine. Freeze frame shows sensor reports -40 degrees.

But the thing is, when I unplug the sensor, the red temperature light illumates on the dash. If the circuit is open then how would it be any different than when it's unplugged?

Do you think the sensor is bad? How could the sensor just go bad like that?

 

I tried shorting the pins for the sensor but didn't get P0117

 

Thanks; I also found this manual which is more relevant:
https://attachments.priuschat.com/attachment-files/2018/04/144922_P0115P0117P0118.pdf

This is a bizarre issue
I used an alligator clip to connect the pins together on the harness end; theoretically this should trigger a P0117, but it doesn't. (IIRC last time I shorted the pins, clearing the codes immediately threw P0118 again.) Temperature light illuminates with this short circuit or with a complete open circuit, but does not illuminate when the sensor is plugged in.

Using the manual I tried checking for continuity from the ECM connector, but I cannot seem to get any continuity between THW/THW and ETHW/E2. The only thing I get is ~1450kOhms between ETHW and either pin on the sensor connector, but that probably means nothing.

 

Spoiler

(There is another coolant temperature sensor in the head that I forgot to plug in. (I was looking at the one on the coolant hose))

 

Yeah, open the spoiler; I thought it was the temperature sensor on the coolant hose above the EGR cooler that was causing the problem, but actually it's the temperature sensor in the engine head that I completely forgot about.