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Sweet T
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Pulled the codes on my family's Expedition (5.4L) and it came up P0171, Bank 1 too lean. My ford CD got ruined, so I don't have that resource. First impulse was a dirty MAF, but wouldn't that cause both Bank 1 and 2 to show lean? Any help is greatly appreciated.
 

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This is what the DVD for the '98 Expedition says - it goes deeper, but starts with this:

DTCs P0172, P0174, P0171 AND P0175: FUEL SYSTEM AT THE CORRECTED FUEL TRIM
Diagnostic Trouble Codes (DTCs) P0171 bank 1 (Cylinder 1) and DTC P0174 bank 2 indicate the fuel/air ratio is too lean. The fuel trim system is at the rich correction limit.

DTC P0172 bank 1 and DTC P0175 bank 2 indicate the fuel/air ratio is too rich. The fuel trim system is at the lean correction limit.

HO2S DTCs Reference list:

HO2S-11 = DTCs P0171 and P0172
HO2S-21 = DTCs P0174 and P0175
Possible causes:

Fuel system

Excessive fuel pressure.
Leaking fuel injector(s).
High fuel pressure.
Low fuel pressure.
Contaminated fuel injector(s)
Ignition system

Spark plugs.
Spark plug wires.
Coils.
Induction system

Air leaks after the MAF sensor assembly.
Vacuum leaks.
Restricted air inlet.
PCV system.
Improperly seated engine oil dipstick.
Base engine

Oil overfill.
Cam timing.
Cylinder compression.
Exhaust leaks before or near the HO2S(s).
Check intake air system for leaks, obstructions and damage.
Check air cleaner element, air cleaner housing for blockage.
Verify fuel level.
Verify integrity of the PCV system.
Check for disconnected spark plug wires.
Check for vacuum leaks.
Verify customer did not run out of fuel.
 
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