Hi I have a 99 5.7 vortec. Iam trying to figure which sensor is on the drivers side of the motor just above the exhast manifold in the head. It has one wire running to a blue connector on top of the valve cover. coolent sensor?

 

It is probably the coolant temperature sensor, the one that sends the signal to the guage ( or warning light).
It would be helpful if you supplied us with more vehicle info.

 

It is a 99 gmc savannah 3500