Check Engine Light (Fault Code PO171)
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Check Engine Light (Fault Code PO171)
I have a 1999 Ford F-150 V6. The check engine light is on and the fault code is PO171, the system is running to lean. I wonder if anybody has had this problem before. I am looking for a fix to the problem. I have been running a injector cleaner through the system regularly but I have been told it could be several things.
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Hotshot
Joined: Aug 2002
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From: BC, Canada
Hi and welcome to the forum 
A P0171 can be caused by a bad O2 sensor but it can also be caused by other things such as a misfire or vacuum leak at the intake manifold, partially plugged injector etc.
I think that if you do a search in this forum and maybe in the 4.2 forum which is a ways further down on the main forum page you'll find more info on the problem.
A P0171 can be caused by a bad O2 sensor but it can also be caused by other things such as a misfire or vacuum leak at the intake manifold, partially plugged injector etc.
I think that if you do a search in this forum and maybe in the 4.2 forum which is a ways further down on the main forum page you'll find more info on the problem.
Hotshot
Joined: Jan 2004
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From: St Louis
An O2 sensor is unlikely to cause a P0171 code
Here is the PCED listing of what can cause the p017X series of codes...
/////////
H41 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.
DTCs HO2S 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).
-- Leaking fuel pressure regulator.
-- Low fuel pressure or running out of fuel.
-- Contaminated fuel injector(s).
-- Vapor management valve (VMV) leaking.
Induction system
-- Air leaks after the MAF.
-- Vacuum leaks (vacuum lines and gaskets).
-- Restricted air inlet.
-- PCV system.
-- Fuel purge system.
-- Improperly seated engine oil dipstick.
EGR System
-- leaking gasket.
-- Stuck open EGR valve.
-- Leaking diaphragm.
Base engine
-- Oil overfill.
-- Cam timing.
-- Cylinder compression.
-- Exhaust leaks before or near the HO2S(s).
l Check intake air system for leaks, obstructions and damage.
l Check air cleaner element, air cleaner housing for blockage.
l Verify integrity of the PCV system.
l Check for vacuum leaks.
l Are any of the above concerns present?
Yes
SERVICE as necessary. COMPLETE PCM Reset to clear DTCs. RERUN «Quick Test».
No
GO to «H42».
//////
Please note that this listing includes the faults for both types of codes (rich/lean) so you have to filter out the ones that don't apply.
Since you only have the P0171, the fault is likely isolated to only the bank 1 (passenger) side of the engine. Most likely causes will be an intake/gasket leak, injector seal leak, or possibly an exhaust leak that is permitting fresh air to be introduced into the exhaust prior to the HO2S11.
Steve
Here is the PCED listing of what can cause the p017X series of codes...
/////////
H41 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.
DTCs HO2S 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).
-- Leaking fuel pressure regulator.
-- Low fuel pressure or running out of fuel.
-- Contaminated fuel injector(s).
-- Vapor management valve (VMV) leaking.
Induction system
-- Air leaks after the MAF.
-- Vacuum leaks (vacuum lines and gaskets).
-- Restricted air inlet.
-- PCV system.
-- Fuel purge system.
-- Improperly seated engine oil dipstick.
EGR System
-- leaking gasket.
-- Stuck open EGR valve.
-- Leaking diaphragm.
Base engine
-- Oil overfill.
-- Cam timing.
-- Cylinder compression.
-- Exhaust leaks before or near the HO2S(s).
l Check intake air system for leaks, obstructions and damage.
l Check air cleaner element, air cleaner housing for blockage.
l Verify integrity of the PCV system.
l Check for vacuum leaks.
l Are any of the above concerns present?
Yes
SERVICE as necessary. COMPLETE PCM Reset to clear DTCs. RERUN «Quick Test».
No
GO to «H42».
//////
Please note that this listing includes the faults for both types of codes (rich/lean) so you have to filter out the ones that don't apply.
Since you only have the P0171, the fault is likely isolated to only the bank 1 (passenger) side of the engine. Most likely causes will be an intake/gasket leak, injector seal leak, or possibly an exhaust leak that is permitting fresh air to be introduced into the exhaust prior to the HO2S11.
Steve
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