What does po171 code mean on a 1999 Ford Explorer 6-cylinder?

Answer 1

P0171 (system bank1) "Bank" is referring to cylinder banks 1 or 2. Bank 1 is the passenger's side and has cylinders 1-4 on it. The MAF (Mass Air Flow) Sensor may be dirty or faulty, or you may have a vacuum leak, especially if you hear a hissing sound. Could be a leak somewhere between the MAF and the intake. Could be a loose hose, bad connection, leaky gasket, etc. Sometimes if you idle the engine and then spray some carb cleaner (carefully) around some of the likely spots you can hear a change in RPMs of the engine. This could tell you where the leak is.