I just bought an 84 4Runner and it needs a little work to pass emessions test, I was told when i got it that it needed a new cat, so today i was looking under the 4Runner at the cat and noticed that it had a probe or something that was bolted to the cat with plug that went into the cab and plugged in under the driver seat. what is this probe and what does it do ?. it also has an 02 sensor that in the exhaust manifold, so i know its not that.

 

My 84 has the same probe. It looks like some kind of temperature sensor. I didn't end up using it after I swapped in EFI from an 85

 

Does anybody know what this sensor is used for ?, if it is a temp sensor then how does it work ?.