1991 S10 2.5 liter - Unkown Sensor

There is a sensor in the top of head next to valve cover. Rear of engine on drivers side. Kind of looks like a temperature sensor. Different type of a connector. The connector has one green wire and it appears to have a place for another wire. I also have a extra Tan wire with a female connector with the matching male piece broke off in the connector.

Engine has 159k miles. Having various engine performance issues. Trying to figure things out.

Appreciate any and all help.

Mallard39

temperature for the ECM, should be oval shaped plug