My 1996 Grand Marquis had the same problem, if you are getting bad fuel economy, it is most likely the O2 sensor (the first one on the driver's side before the exhaust system). If you have a code for both bank one and two, then you probably need to clean or replace your mass air flow indicator.
Bank one is "LEAN" - get a good bottle of Fuel Injector Cleaner and see if that helps.
Fuel system lean bank number 2Fuel system lean bank number 2
what does system to Lean Bank 1 on 740iL 2001 means
p1131 = lack of HO2S11 switches - sensor indicates lean
P0171 = System too lean (Bank 1) Look for a vacuum leak before you buy any parts.
P0171-system too lean bank 1 and P0174-system too lean bank 2
Bank one fuel system lean. Test fuel pressure, check for vacuum leaks,
Bank #2 fuel system lean.
P0171 System too Lean (Bank 1)
P0171, system too lean (bank 1). P0174, system too lean (bank 2). Start with checking for a vacuum leak.
This is an educated guess! It probably regulates the amount of oxygen in the fuel system. it senses the O2 level in the exhaust too much means lean too little means rich
Trouble code P0171 means: Fuel Trim System Lean (Bank 1) Trouble code P0174 means: Fuel Trim System Lean (Bank 2)