Check engine light on

What is your question?

Check engine light is on (steady light) and got gasoline a couple of days ago. Told that it might be the gas cap not turned enough times, but light came back on for mechanic as drove it. Diagnosis was unclear, but thought it might be fuel pump related. Mechanic stated he hears a hiss sound when under car. Took to Auto Zone and diagnosis was throttle sensor, but mechanic says not the sensor. Please advise as budget is extremely small! Thanks!