[Occam's Chainsaw]

My 2003 Sienna seems to have three temperature sensors. Two are on the upper housing where the radiator cap is. One seems to be a resistive sensor with two terminals, and reads 2200 Ohms when the engine is cold. The other is a single terminal, and I assume it's the sender for the front panel gauge.

There's a third sensor at the bottom of the radiator. I can't find any mention of it in my online search. What's it for? Is it a resistive sensor or a switch? How do I test it? Does it have a part number? It doesn't take the same plug as a replacement that I bought for the upper sensor, and reads about 1.7 MegOhms.

I'm on the trail of why the fans run when the AC is running, but not when the AC is turned off, following an over-heat incident.

 

[Occam's Chainsaw]

Updated info. I now believe it's a temperature switch. According to one schematic that I found, it should be closed when the engine is cold. It's open. This switch is mentioned in the Haynes manual, but I can't find hide nor hair of it at any of the websites, such as AutoZone and Rock Auto. Tonight I will try shoving a paper clip into the connector and see if it makes the cooling system work. But I'm on the fence about fixing it, because the lower body of the radiator is plastic, and I'd hate to crack it while trying to remove the old sensor. It might be a mechanic job at this point due to not being able to identify the part, and not wanting to crack the radiator. At least, if I'm right about the symptom, it will be a relatively inexpensive mechanic job.