I have a 2002 F-150 with the 4.2l engine. I had the p0171 and p0174. The truck runs and drives fine but it keeps throwing those codes. I replaced MAF, Upper intake, injector O-Rings, fuel pump, fuel filter, vacuum hoses, pcv valve, EGR valve, EGR pressure sensor, spark plugs, and probably a few other things I cant remember. When I got it it wouldnt want to run (kept stalling out and shaking) now it runs and drives perfect. My question is if the PCM wasnt reset, would it still keep throwing those codes because of the computer being used to the old conditions?
If you have any other thoughts on what it could be, please let me know. Its in the shop until I get paid Friday. They just replaced the Mass Air Flow sensor. Before that it drove good but was a little rough and shook a little at idle.
Edit: I have around 140000 miles on it

 

The bolts that hold the upper intake to the lower are known for causing vacuum leaks. They are called isolator bolts. If the PCM wasn't reset after fixing the cause of the code then the check engine light will stay on until it doesn't see a fault for 50 key cycles. It doesn't get used to anything.

 

I also replaced the isolator bolts when I did the intake.
would the codes keep coming back after being cleared by code reader?

 

Check the oil tube where you pull out to check how much oil is in the engine. The tube or the o-ring could be bad and both will let air in and give you a lean reading.
Old or clogged fuel filter, dirty injectors, intake gasket, MAF sensor, air filter, air temp sensor, vacuum lines, EGR system, air tube coming to throttle body having a hole in it or whatever, these are a number of things that can cause this code.
As for the PCM, it could cause it, but not likely. Clear the codes and see what happens as the PCM should clear after so many starts and drives by itself.
The odds of both O2's being bad are high.

 

I didn't think even think about the oil dipstick O-ring. I already replaced the fuel filter, intake gasket, and MAF. When I did the intake gaskets I blew the fuel injectors out with carb cleaner and compressed air. The air filter didn't look bad so I left it in there. I used propane on the vacuum lines and didnt see any difference with the way it ran. When I had the intake off I cleaned what I could out of the EGR ports in the intake.
When I clear the codes manually with a code reader they keep coming back within 20 or so miles.

 

The odds are actually quite low. The O2 sensors do not cause "lean" readings, they report the condition they see.

You need more than a simple "code reader" to properly diagnose the issue, you need a scan tool (or equivalent) that can display both freeze frame data (so the fault conditions can be accurately identified) as well as live data.

 

I have the craftsman auto assist that does live data for fuel trims and stuff like that.

 

Then use it to pull the freeze frame data first. Having the tool but not using it appropriately does not assist you.

 

Right there with you 70 F100Long Bed. I made a ton of money at the dealer too and got my masters cert in 1998
I have the tool too and don't sweat the hard stuff either (those engineers told us a story for sure on those early 3 valves)
Now the balance shaft in those Mercedes are a hoot
Makes a 5.4 with the phasers look like a toy and is way worse than the coyotes or 4.0 SOHC

 

70f100longbed,
Mercedes M272 and M273 balance shaft chains tensioners oil separators
4x4 trans out, front axles out and oil pan off. Intake off, valve covers off centrifugal oil separators for the pcv the works. You should come and help. The wife is a good assistant but OMG.
I have taken about 6 months off on the job trying to come up with enough whisky and or grenades

 

Don't you get a kick out of this site like I do seeing some of these guys fight it
without all the stuff that makes it easy for us
Hell having a capable scan tool or 4 of them and knowledge is the key
I do not use freeze frame data much except for intermittents and just examine pids closely and decide what needs to be done

 

On Fords, before chasing vacuum leaks always check the MAF. If you replaced the MAF, did you use Motorcraft.? Ford is fussy and it makes a difference. A common problem with aftermarket MAF's is they don't generate the correct barometric pressure readings and it totally messes up the air / fuel ratio which throws those codes. Make sure the MAF is giving the right barometric pressure readings before chasing other issues.

If the MAF is all good then yes, get the freeze frame data and look at the vehicle speed when the codes were activated. If the codes activated when the vehicle was traveling then it is likely not a vacuum leak. If it happened at low speed or idle the it is likely a vacuum leak.

 

The OP's engine does not have a BARO sensor. That said, the MAF sensor, if its output is skewed, will certainly cause fuel mixture concerns and it does remain on the list of suspects.

This is the exact reason for checking the freeze frame data. The person doing the diagnostic must be able to determine if the fault occurred under a high vacuum condition (usually a vacuum leak) or a high load condition (usually an inadequate fuel supply).

 

The connectors had issues too so whether you check freeze frame or not you had better interrogate the connector while checking the maf pid

 

The BARO readings on this model come from the MAF sensor.