As said its a cat temp sensor
These little boogers can cause running issues, lack of power etc similar to a faulty knock sensor.
Troubles caused when people can't unscrew them due to rust so they cut them off and tuck them up under heat shield or where ever, the outer braiding can short with inner and sometimes triggers the hot dog light so then does as thermal said in earlier post.
If you have one which has been cut off then you need to completely unplug it under driver seat then it can cause no grief
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