P0171 means that the bank 1 is running too lean which can be caused by either unmetered air entering the engine or by not enough fuel being delivered.

Very often lean codes are set due to vacuum leaks allowing unmetered air to enter the engine, so try to spray a little bit of carb/brake cleaner around the intake manifold gaskets and vacuum hoses and listen for a change in engine rpm.

Another thing to check is the bank 1 02 sensor to see if it is switching properly. If the oxygen sensor isn't switching properly, the PCM could interpret this as a lean condition and set the code.

 

I was under the impression that if it were the O2 sensor, it is supposed to throw it's own specific code (P0133 I believe). I can take some carb cleaner and try that method once this storm clears up.

Also, I just noticed that the clamp for the air filter hose that connects into the intake manifold was missing!? No idea how that happened.

Just clamped that sucker back down. Could excess air have been entering through there, leading to this bank 1 lean issues? Highly doubt it, figured it would have thrown both P0171 and P0174 if that was the case.

Also, how do I diagnose the EGR? Still have a sneaking suspicion that it's the culprit.

 

Did you also check to see if you had a pending 174? Did you check fuel trims on each bank? It is possible for a vac leak to cause fuel trim on one back to go just high enough positive to trip a lean code, while fuel trim on the other bank doesnt quite reach the threshold for setting a code.

You are right.... 171/174 are never due to a failed 02 sensor. They do set their own codes for failure to switch, which can be due to either a VERY lean condition due to a large vac leak or a failed 02 sensor.

 

I did not check the fuel trims, as I don't believe my scanner has that capability. And no I had no pending code for the P0174. I erased the codes after re-clamping the air filter hose to the intake manifold and replacing the connecting hose from the fill cap neck to the air filter hose....however I'm expecting it to resurface.

 

The main idea to understand is that P0171 is a lean code so that means that bank 1 is too lean by either too much air entering the system (vacuum leaks, MAF, EGR stuck open) or not enough fuel delivered (faulty injector, low fuel pressure, fuel regulator problem, etc). P0171 and P0174 frequently occur together, but it is not always required. Sometimes there can be a lean condition, and not have any codes at all so that's why it's important to check each possible cause and eliminate one by one. Evidence will always lead you to the problem.

A failing 02 sensor does not always set a code, if it is stuck lean then either there is a vacuum leak or the 02 sensor is faulty and no longer switching rich-lean. The PCM interprets the lean condition and sets the code, so while the sensor may or may not be the problem, it is worth checking to eliminate it as a possible cause. If the 02 sensor is switching properly, then you can rule it out definitively and look elsewhere.


Also most importantly of all, the engine will not run properly without the MAF. When trying to diagnose fuel trim problems, it is imperative to have a good MAF sensor. Good working used MAF are readily available at most local salvage yards, and it's much better to replace than to drive around with it disconnected.

 

V8SHO, thanks for your feedback.

Diverging from the original code issue, but still relevant to the conversation, I'm a little hesitant to install another MAF.

We had the car's transmission replaced by a very reputable shop. They provide a 2 year warranty and are not gold-diggers. Less than 3 months after the new transmission, we found ourselves with yet another faulty transmission.

According to their shop, they isolated the problem to the MAF causing havoc with the transmission...so it wasn't their fault that the tranny died, but yet they were stuck with the bill of rebuilding the transmission a second time. They instructed us to leave the MAF unplugged or risk frying the tranny again.

Why would this happen? If I replace the MAF with a brand new one, do you think it would fry the tranny again? Still very skeptical.

 

The lack of power caused by the faulty MAF could have affected the engine load which the PCM perhaps tried to compensate by adjusting the shift points. This ultimately could have lead to the transmission failure. However replacing it with a good working sensor should not cause any problem, but I can see how this would cause a concern about the transmission warranty. Although, the fact that the engine wasn't running right is what caused the transmission to fail. So leaving the MAF unplugged isn't really the right thing to do, and honestly it was not right for the transmission shop to decide to do that. They fixed the transmission but didn't bother to fix the underlying cause.

 

Makes sense. I'll pick up a MAF sensor from Autozone today and replace. We'll see what happens. The P0171 code still has yet to come back as well and we've driven a good 50 miles thus far. Don't know how long it takes to throw the code but it seems to be holding out if there is still an issue.

Stay tuned.

 

DO NOT replace your MAF until you have checked the grams per minute or lbs/hour air flow using a scan tool and compare it to published specs. Also, Auto Zone electrical parts are in general TOTAL JUNK. NEVER use ANY AZ electrical replacement parts!!!!! You will find MANY members here who will back me up on this.

NEVER throw parts at a problem. Perform a systematic diagnosis to find the root cause of a problem.

 

DO NOT replace your MAF until you have checked the grams per minute or lbs/hour air flow using a scan tool and compare it to published specs. Also, Auto Zone electrical parts are in general TOTAL JUNK. NEVER use ANY AZ electrical replacement parts!!!!! You will find MANY members here who will back me up on this.

NEVER throw parts at a problem. Perform a systematic diagnosis to find the root cause of a problem.

I do not believe MAF can cause the tranny to fail. Tranny is more robust than that. Bad MAF could cause bad performance and code, but they seldom fail. I agree with you, I would trust a JY unit first. I have JY units for the Lin and my Sables. Cleaned, switched them and makes no difference at all. So, I have spares.
Not unusual for a shop to look for a "scapegoat".

-chart-

 

Update:

New MAF yielded the same problem as it had in the past...made the transmission act very goofy, taking forever to shift into 1st, along with a host of other symptoms. Just removed the new MAF and will return tomorrow. This issue goes beyond simply replacing the MAF.

As for the P0171, code still has not come back. Sprayed brake cleaner along the intake gasket and got no change in RPMs. Will keep everyone updated on whether that code comes back.

Still boggled about the MAF screwing with the transmission. Guess that tranny shop was right, I just don't know what to do about it.

 

Did the trans shop tell you what failed in the transmission they installed. A well rebuilt trans should be able to deal with even a heavy footed driver longer than yours did. That warranty is a scam if they do the diagnostics and blame an unrelated part. Think about the money they just made by blaming another part. I bet they gave you a labor discount since they installed the first one just to get you to reuse them.

 

Automender, I think you misunderstood....they incurred the entire cost of the replacement transmission. They did the work and narrowed down the cause of the failed rebuild to the MAF, but still covered the second rebuild under the warranty. I didn't pay anything for it.

My brother has fleet cars in his line of work and uses them religiously. They're known for doing good work...no scams here.

 

Well given that installing a new MAF didn't do anything, that means there is definitely a problem elsewhere causing the performance issue because an engine just will not run properly without the MAF. It makes more sense now that the shop didn't bother to fix the problem because they probably couldn't figure it out, but they left the sensor unplugged as their only solution.

Depending on how far you want to pursue the problem, it might just be best to leave it alone at this point, because without access to the right tools and resources it's impossible to say what could be the underlying problem.

 

V8SHO, the MAF actually worstened the problem....it didn't just "not do anything". There must be a secondary issue that makes the MAF screw with the transmission. I just have no idea what that issue is. Needless to say the car isn't worth investigating that. We will continue to drive with the MAF and as far as the P0171, it still has yet to return.

 

Nope. The transmission shop is pulling things out of the air to try to explain that.

 

Have the upstream 02 sensors been replaced? After about 100k miles they become lazy (slower to react) and can cause 171/174 codes. Check the wiring connector. Oem Motorcraft sensors are green, same as the wiring harness. Several have replaced old sensors, had the code go away and improved drivability and mileage.
171 is a bank specific problem. A vacuum leak of slow sensor is most likely. The mass air sensor measures airflow for both banks and not likely a culprit.

 

Have the upstream 02 sensors been replaced? After about 100k miles they become lazy (slower to react) and can cause 171/174 codes. Check the wiring connector. Oem Motorcraft sensors are green, same as the wiring harness. Several have replaced old sensors, had the code go away and improved drivability and mileage.
171 is a bank specific problem. A vacuum leak of slow sensor is most likely. The mass air sensor measures airflow for both banks and not likely a culprit.

- Intake air leaks
- Faulty front heated oxygen sensor
- Ignition misfiring
- Faulty fuel injectors
- Exhaust gas leaks


Read more: P0171 Ford - Fuel Injection System Too Lean Bank 1 - OBDII Engine Light Trouble Code | AutoCodes.com

-chart-

 

at this point, you really need a OBD2 scanner that will do live data. Harbor freight has one, or you can get one of the Blue tooth PC based ones.
You need to look at the fuel trims both short and long to see what is happening on both banks. both with and without the MAF.
Also, what is the fuel rail pressure? Is the vacuum line to the fuel pressure regulator intact if you have one?

You also need to closely look at all VACUUM hoses under the hood for any cracks or irregularities. replace any that look bad.
replace the PVC donut gasket if its too loose..

Its not beyond possibilities that you have a partially plugged cat too.

 

This sounds like a 60 minutes episode.
A bad mass air flow sensor won't cause transmission failure. Disconnecting to keep the transmission from failing is also the wrong thing to do.
If the 171 code returns, remember this is lean, bank 1, not both banks.
A loose exhaust flange or slow 02 sensor, wiring or bank related vacuum leak is most likely, this includes the o-rings on the injectors where they go into the intake manifold.
Misfires are more likely to set a cylinder specific code unless they are borderline. Short and long term fuei trims are needed to nail down injector problems.
It's possible the 171 code was caused by a loose connection which has been resolved.

 

If our MAFs have the air intake temp sensor built into them then they absolutely COULD cause transmission issues. This is a pretty common GM issue on newer cars. Bad wiring or bad intake temp sensor can cause tranny to run all funky and at abnormal pressures. I had a Saturn l200 with this issue. Could not figure out the HARD shifting and kept ignoring the intake temp sensor code until I got the trans fixed. Eventually decided to take care of the sensor issue (wire broke inside insulator) and everything was perfect again. I know this post is old, but the first thing I would have checked would be intake air temp with MAF plugged in