I have a leak only when the water is turned on. It appears to be coming from the middle/stem,not the handles. The leak is underneath the sink. The handles come off with an allen key but there is no such key for stem. There are 3 plastic pieces underneath the sink securing the handles and stem to sink/countertop which move but I cannot undo them nor can I remove the stem from the top.: confused:

You shouldn't have a 'middle stem' on a bathroom sink. Is it the spout? Is this an 8" or widespread lavatory faucet? Is it leaking above or below the sink? Is it leaking on one or more of the connecting hoses?

Wrong term. It would be the spout then:o It is about 8" long, curves down. The leak is below the sink and does not appear to be coming from either of the pipes, not hoses. It seems to generate from the middle, where the drain stopper handle is located. I can't figure how to get the spout off, l assume there is some sort of seal.

Remove the drain stopper pull handle and peek inside the hole. Does it appear to receive a long allen wrench?
You possibly have a defective spout or are simply missing the areator and it is running down the front of the spout. Wrap your hand around the spout and then turn on. Does your hand get wet?