What is a purge sensor for the gas tank?

Answer 1

You are probably referring to the purge solenoid. It is used as part of your vehicles Evap system. The fuel in your fuel tank is constantly giving off vapor. This vapor would cause a huge amount of air pollution if allowed to just go off in to the sky. The evap system allows your engine to burn these vapors in the engine while driving instead of allowing them to get in to the air.

The evap system is very complex and contains many parts. The purge selenoid opens and closes allowing engine vacuum to suck in the fuel vapors. It only operates when the engine computer sees the appropriate conditions are present.