Hello. Here is the situation. I have an 01 F-150. Check engine light came on. PO171 & PO174 Code read lean bank 1 and 2.
I have changed the fuel filter.
Replaced the mass air flow sensor.
Reset the code and it came back on.
Then I took it to my mechanic.
He checked the throttle body gasket - no leaks.
He then thought I might have gotten a bad mass air flow sensor replaced it - light still came back on.
He then did a smoke test to see if there were any leaks - result no leaks and the light came back on.
He then placed gauges to check fuel pressure - Pressure was on the low end.
He then replaced the fuel pump (we were hoping that would fix the issue) - He ran the truck for a few and you guessed it the light came back on. Same code throughout all these actions.
At this point he is at a loss of what could be causing the code for lean banks 1 & 2.


Has anyone on here ran into this issue or have any idea what could still be causing this light?


Thank you all for your help or suggestion.


Lowgear

 

When was the last time you changed your pcv valve? Or even checked the pcv hose ?

 

This. I was running into the same problem. I replaced the PCV valve and hose. I also replaced some small rubber vacuum hoses that were dry rotted. That was good for about 15,000 miles and it recently came back on. It ran noticeably better after replacing those too. The truck has 236,000 on it and I am probably replacing it sometime this year, so I am not really motivated to try and find the problem. I give the MAF sensor a bath every now and then and reset the light and keep driving.

And OP, this is a super common problem. If you search, you find tons of info on it.

 

I haven't ever checked the PCV valve or hose. I just bought this truck of course with no lights on and on the way home the light came on. Do you think that could be the problem? Thank you for the reply.

 

Yes could be. The hose connection tends to dry rot where it connects on the back side of the motor causing this problem. Again very common problem and a google search will yield tons of options. I don't remember them all.

 

Gentlemen thanks for the replies. The PCV Valve and hose are the one thing the mechanic didn't check. I'm going to go over there and talk to him now. Fingers crossed that this works. I mean we have replaced all the other parts... Thanks again.

 

Sorry your having troubles. My 02 did the same thing and we went down the same road you are. The smoke test came back negative it wasn't until we sprayed some starter fluid around the intake that it finally showed up. My fuel pressure was also low but that was due to fuel filter that needed replaced. If I remember right I was around 30psi which is on the very low end, the filter helped that but I still suspect I will be doing a fuel pump soon.

 

Could also be the O2 sensors. I went through multiple issues similar to yours.
Bought my '02 in July '16.....
Replaced the TPS, cleaned the MAF sensor, replaced left manifold gasket, PCV hose, spark plugs & boots and everything except fuel injectors on the fuel system....unfortunately all needed attention or replacing. It wasn't "replace parts until I got it right".

Then all 4 O2 sensors went bad showing the same lean codes. Should be all set for the next 100k miles!

 

These DTC's are the most common and yes, can be very difficult to troubleshoot without a ton of field experience and understanding. For these vehicles, it's the PCV system that is most common. Simple fix, so go there first, then hit the MAF o-ring and check the air box assembly since both banks are positive for lean.

As mentioned, degraded O2's sensors can contribute. As mentioned again, 100,000 miles plus is when this can occur with today's petrol and sensors.

With all the experience gained over the years (decades) I'll post up the best website for a quick understanding and quick troubleshooting that you can have in your arsenal. Bookmark it! -

http://www.fordscorpio.co.uk/fuelmonitor.htm

I've gone there ^^ before the online service/oasis link and info. Still do when I'm after a refresher lol.

 

Invest 30 bucks in a code reader and the Torque smart phone app. Monitor your long term fuel trims and see when the problem occurs. You can check your O2 sensors with the app also.