recently rebuilt engine after intake gasket leak, but won't start. couldn't find where this one connected to, wondering if it has anything to do with not being able to start it? photo is attached

 

looks like it could be a knock sensor plug - look on the back of the riders side head between the firewall and the head - usually one in the drivers head also - it sits between the spark plugs - and with a knock sensor unpluged it would still start

 

My first thought was knock sensor as well but I will change my mind to coolant temp sensor. I'm pretty sure the knock sensor will be mounted on the block and not the heads. Then again I was going to call you out on not having all the plug wires on yet... just going by the picture.

 

some are mounted in the water jacket on the block - on my 92 that i built my avatar out of and the 97 LS that i just gave away had them in the heads - riders side screws into the back of the head with a regular bolt thread - drivers side screws into the water jacket in the head with pipe thread