My check engine light came on and had it scanned at Auto Zone. The code was PO171, which means bank one is too lean. I assume this is an oxygen sensor but not sure which one. Any help and suggestions are greatly appreciated.

Thanks, Mike

 

First what are we working on here, I assume a 97 or newer 4.2, 4.6, or a 5.4 and what mileage? From there, you got to stop and rethink here.
Just cause you have a code 171 (lean) doesn't mean automatically that you have a O2 bad. It ccould be that is just how the PCM was able to pick up the problem. Let me give it to you in this example to help explain. Let's say you have 97 F150 thats has a partially clogged injector or two but otherwise are OK, the PCM has no way of knowing it's clogged. In turn that cylinder gets less fuel but still the same amount of air so it burns lean. The only way the PCM knows something is not right is when all this pure air gets to the O2 it sees it as a lean condition and triggers a code 171 for that engine bank".
I'm not saying it isn't an O2 because it very well could be just an O2 problem. You can't be sure unless test it either with a scanner or voltmeter or you buy a new one and it fixes the problem. I am just warning you on how these OBDII system can operate. Lots of the engine sensors and pieces can cause a lean code is all I'm saying.

 

Thanks for the info. I meant to add that my truck was a 1997 F-150 (5.4L) at the end of the post but I got in a hurry and forgot. I guess I need to do some more homework according to your expertise. I do have the Haynes manual for my truck and it describes how to check for faulty O2 sensors. I am really trying to avoid dealer assistance because they always want to tear me a new one for minor things. Anyways, if you have any other suggestions I am open for anything. In the meantime I guess I will use what I have and try to come up with something. By the way, the check engine light hasn't came back on as of yet.

Thanks, Mike

 

check for vaccuum leaks. inspect all the vaccuum hoses and especially the rubber elbows on them. there is a breather that goes from the driver side valve cover to the air intake tube(i believe on the 5.4), check where it goes into the tube before the throttle body and make sure it's a tight fit. what happens is oil vapors cause the rubber to swell up and it lets un-metered air into your engine. it is probably most definately not the o2 sensor that is bad. I think the egr tube that goes from the passanger side valve cover to the back of the engine is known to swell/deteriate and leak also. Carefully inspect every inch of hose that you can and see if you find anything.

 

I checked both hoses and found the passenger side PCV valve laying on top of the head cover. I sure hope that is it because those O2 sensors look like a pain. How could that PCV valve cause a lean condition? You know something and I hope you can share it with me. You are the man 02 RegularCab4x4!

 

wow! you found the egr valve laying on top of the valve cover? if you found the egr valve laying there, not hooked up to the rubber/plastic egr hose,then, what happens is the engine is sucking in air where it is suposed to be sucking in vapors(already burned gasses). this causes the 02 sensor to read lean. generally if the 02 sensor is bad it will give you a code for "o2 1or2 is slow to respond" or something of that nature. now i am a little puzzled as to why your pcv valve was laying there. did you have it into the dealer for repairs lately? I would put a new pcv in it while you're there and a new hose if the pcv dosen't fit tight, follow that hose all the way to the back of the intake and make sure the rubber elbow back there isn't swelled up and leaking also. remeber to un-hook your battery(negative cable)for about 10-15 minutes to clear the code.

 

Well, that could be the problem. If the PCV sucked in to much air I guess it could cause that. ALso, it sort of a known issue that the PCV hose like to crakck and get brittle at the very back where it runs into the TB. I know mine is brittle but i have yet to replace it. I thought you could just use some rubber hose but someone said it changes sizes, so it's either rig up something or go buy a new PCV valve hose assmebly (which is like $20).

You can use propane, starting fluid, wd40, or carb cleaner to help check hoses. If you spray a hose and idle changes then theres a leak. Just be careful.

 

Actualy, I had a new long block installed recently due to internal damage from a blown spark plug in the #3 position. It was probably left out during the engine exchange I would guess. I will check that hose for swelling and cracking also. Thanks for the info and have a great Thanksgiving.

Mike

 

The Ford Service Manual gives this as what to check

Excessive fuel Pressure
Leaking /contaminated Fuel injectors.
Leaking Fuel pressure regulator.
Low fuel pressure or running out of Fuel
Vapor recovery System
Induction system : - Air leaks after the MAF.- Vacuum Leaks PCV system. - Improperly seated engine oil dipstick.
EGR system: - Leaking gasket - Stuck EGR valve - Leaking diaphragm or EVR.
Base Engine: - Oil overfill - Cam timing - Cylinder compression. - Exhaust leaks before or near the HO2Ss

Do yourself a favor and part with the money for the service manual. It will prove itself more then useful. Maybe a ODBII scanner, I have never trusted Pep Boys for scanning code, that is why I got one. <g>

Hope you got it with the PCV valve.