Hey all,

My check engine light is on and I took it to autozone and the free diagnostic testor said PO bank 171 and 174 is too lean. The guy recommended buying 2 O2 sensors and said it would for sure fix the issue. Another man at Oreileys said its probably the throttle body...
I went to another shop while in denver (right when the check engine light came on) and they did a full diagnostic and said it needed some type of gasket because mine was starting to tear..

In arizona heat the truck runs great with no issues..
last year while in denver cold it would die out if it was too cold.. I would have to hold my foot on the break and gas at the same time so it would sit and idle -while driving around and giving it gas it ran fine-

So my question is, does 171/174 automatically mean replace these sensors?
I dont think the sensors have ever been replaced when my dad owned it (miles 0-70,000) and they definitely havent been replaced since Ive had it (70k-110k currently)

it is a 2000 f150 4x4 5.4l

Thanks for any help.
Ryan

 

AutoZone guy - Strike 1

Oreilly's - Strike 2

Mechanic - Base hit. That guy knew at least that O2 sensors don't cause these problems, it's usually a vacuum leak. PCV valve elbows are most common, followed by some other leak in the intake tract. Since it's most apparent in cold climes, it's probably something that's shrinking in the cold and an intake gasket is a logical possibility.

 

The 171/174 codes means the air/fuel ratio near idle is incorrect.
The sensors are what detects it, not the cause.
If these shops would use a Scanner to look at the fuel trims they would see this and not guess. The long term tables would show over 25% shift. The short terms could be anything if the OX sensors are still switching.
Actually the 171/174 codes are there to tell of a leak or anything that upsets the air to fuel ratio near idle because it affects drivability at idle so much at stops and under varying engine loading near idle speed and can even cause some harshness with the transmission at times as the PCM tries to recover the idle from a large rpm change just after put in gear or coming to a stop.
If you go back to the tail pipe and listen , you will hear the upset in A/F ratio making cylinder firing uneven.. Same as you would hear on a carbed motor with misadjusted idle screws.
What you would hear is not a misfire b ut uneven cylinder pressures.
To double check this, use a Scanner to look at mode 6 test 53 at the misfire monitors. If the motor runs smooth above idle, the monitors will read zero or very low misfire rates proving it is not misfire causing the roughness.
Another indicator is jabbing the throttle off idle will smooth the motor out then it will return to rough idle as the OX sensors catch up from the response of the jabbing.
Looking at the idle speed on a Scanner will also show the rough idle that may not be seen on the dash tach.
Above idle, all this has little to no effect because the proportion of leaking air upset is greatest at idle but becomes a "very small fraction" of the total at higher road/engine speeds until you come to a stop again then it all come back to be a pain.
So you, see a lot of shops don't look and don't understand the issue in any depth to make any sound judgment .
This is why you get the run around from guessing and mis-interpretation of the codes from various sources.
You have to roll with it until you realize it at some point.
Be sure the hoses, rubber fitting are all good, then a smoke test to find the leak where you can't otherwise at gasket seal locations etc.
Very often spray testing does not locate the leak.
What you have at this point is an engine that is already running to rich due to the leak from "PCM correction" trying to correct itself. Adding more test fuels through a leak don't help or result is any reactions.
If the motor were truly lean from "missing fuel" then it would react to leak test fuel.
Good luck.

 

Lots of information here, thank you 2 for your time.

Ill definitely skip replacing the 02 sensors for now but will get the intake gasket replaced considering it needs to be done sooner than later. If that does not fix the issue I will see about the info Bluegrass mentioned.

 

Additional info:
If your sure no rubber or connections are at fault, try Brake Clean spray along sides of each bank and observe idle speed for changes.
Lacking any reaction, aim the spray onto the separate center plenum mating surface areas both front and back through the out side legs of the runners both sides.
If there is a leak you may not hear a huge idle reaction but with a Scanner hooked up to watch the fuel trim tables reacts instantly.
Remember if both 171 and 174 are present the fault is common to both banks.
That about eliminates both OX sensors, any injectors, any ignition faults and mass air meter unless they were present before the 171/174 codes came about.
A Mass Air meter fault is electrical in nature not an air leak. It will react differently as read out for fuel trims reactions to revving the motor. A completely different set of conditions
Good luck.

 

Spray rarely works unless you are lookin at the short term fuel trim or the O2 sensor voltages with a scanner. The PCM is usually too fast to correct otherwise and you won't hear any difference.

 

After 215K miles I now have a 171/174 code on my truck..
With a Scanner attached and looking at it while spraying the center bottom cover, the trims instantly reacted to the spray as well as the idle speed changed.
This is repeatable every time spray is done to the same area.
There was no reaction in any other part of the system at hoses, outer head ports gasket area or rubber fittings.
Fuel pressure is at normal levels and controlled by the regulator using a hand pump.
Ox sensors are fully active.
The fault had to be positively identified before taking anything apart or it's just guessing because it requires taking the whole intake off the motor for repairs..
The bottom center plenum is a cover with a set of rubber bung like gaskets.
I just by coincidence have the same trouble at this time and able to report how it acts, how it was found and how the dynamics of it works as observed on a Scanner.
This issue may seem simple on the face of it but locating the cause takes some work and observations with a Scanner after hoses and fitting have been ruled out.
How shops or stores can make flash judgments is beyond comprehension without diagnostic work to locate the source.
The roughness is the air-fuel ratio out of limits causing the idle to wander, the PCM trying to get hold of it and and the fuel pressure wanders as well due to the vacuum signal it sees that changes the fuel pressure during a drop in RPM.. As well the ignition timing also wanders all over along with it. No wonder the idle is rough and wanders as a result of all these actions that are normally absent.
The PCM is trying to restore an operation it cannot do from an outside air leak.
It's a hell of a system to work like this. It's among the best there is and has to be understood to be appreciated for what it is able to do.

Good luck.