I'm replacing my starter and I noticed a sensor that goes into the eng block right in front of the starter. What is this? I would like to replace it while I got the starter out. Also I put a socket to it and I'm truning counter-clockwise but it won't loosing. Any advice.

 

It's the Knock Sensor.
Try a big wrench and some penetrating oil.
If you don't have the right size wrench you should be able to get a cresent wrench on it with the starter out.

 

Thanks M&M. What does the sensor do? Knock as in a ping. I am getting some ping and I use premium gas.

 

If it senses a knock or ping it commands the ESC module (or ECM) to retard the timing until the knock goes away. It will then slowly bring the timing back up.