Hello all, my first time posting on here. This site has been a great help.
I have a 99 F250xl with 5.4. I just got a code reader and Iit tells me
P0171 and P174 (system too lean bank 1 and bank 2). Could someone tell me what that means and what my next step should be

 

I just fought both those codes a few weeks back. After replacing the O2 sensors I still had the issue. It ended up being the vacuum line to the fuel pressure regulator for me. I would recommend searching and finding this problem pretty quick, as lean conditions can cause damage much more quickly than rich conditions.

Also, here are a few other possible causes

PO171
System Too Lean Bank 1
The PCM monitors the adaptive fuel table to ensure fuel control system components do not vary from their nominal values. The fuel control strategy within the PCM uses the adaptive fuel table to compensate for normal wear or aging of fuel control system components. The code is set when the PCM can no longer compensate for additional changes in the fuel system.
Obstructed or dirty fuel injector(s
Suspect EVAP EVAP canister purge solenoid.
Leaking exhaust system.
This DTC may be caused by :
Suspect Low fuel pressure.
Crankcase Ventilation System
Suspect air intake system leak.
Suspect engine vacuum leak.
Suspect HO2S11 circuit.
Suspect PCM.
A blocked fuel filter will restrict the flow of fuel to the pressure regulator. If the fuel supply to the fuel filter is O.K. the fuel filter may be blocked.

PO174
System Too Lean Bank 2
The PCM monitors the adaptive fuel table to ensure fuel control system components do not vary from their nominal values. The fuel control strategy within the PCM uses the adaptive fuel table to compensate for normal wear or aging of fuel control system components. The code is set when the PCM can no longer compensate for additional changes in the fuel system.
Obstructed or dirty fuel injector(s
Suspect EVAP EVAP canister purge solenoid.
Leaking exhaust system.
This DTC may be caused by :
Suspect Low fuel pressure.
Crankcase Ventilation System
Suspect air intake system leak.
Suspect engine vacuum leak.
Suspect HO2S21 circuit.
Suspect PCM.
A blocked fuel filter will restrict the flow of fuel to the pressure regulator. If the fuel supply to the fuel filter is O.K. the fuel filter may be blocked.

 

I have a PO175 system too rich bank 2. I changed the O2 sensor upstream on the driver's side and cleared the code. Two days later the same code shows up. The truck seemed to run ok, but now I think it may be miss'in sometimes. If it was a leaking injector, would'nt I have a miss fire code?

 

I hate CEL'S...always could be anything...I miss the old days before all this computer stuff on vehicles!

 

Check the pcv molded line that connects the pcv to the throttle body elbow thes are known to fail and will cause a vacume leak and set codes. also as the leak gets worse the engine is going to run rough.

 

I, too had these same codes on my 4.2L 4x4. Before I knew about this forum, I took advice from Advance Auto and replaced the 02 sensors, which were a pain to take out! Needless to say, that didn't do the trick.

My problem was an exhaust leak between headers and "y" piping before the cats. The nuts & bolts were completely rusted off and caused the leak. Was really loud when first started until she "warmed up" and the expansion of the pipes would quite her down.

Took it to Midas and they showed me the gap as well as gaps at header mounts. They replaced 'em (headers) and now the truck is quiet as hell AND the CEL is off.

Take a look and make sure you don't have any exhaust leaks. Hope this helps.

 

I had same codes on my 99 4.2, after checking all the 02 sensors, replacing plugs, replacing fuel filter, air cleaner, cleaning MF, finally found a vac leak in pcv elbow near the egr (center front by throttle body). Changed it and reset computer......Runs like a champ now......
CHECK FOR VAC LEAKS FOR SURE

 

Just came back from the shop, $299.80 later. Cleaned mass air sensor, they changed my spark plugs and checked for vac leaks and none found. I think it is a bad injector. Oh, and they changed my brand new K/N air filter without asking and they said they are no good for the truck.

 

update: cleaned MAF sensor, checked around for leaks found none. Light is out and its day four. Paid 150 buck for the code reader and not the dealer. Thanks for the tips.