P0173 Fuel trim malfuction bank 2

[youngvito]

i have a 96 328i my check engine light came on my OBD read P0173 whats dose this code mean and how can i fix it?

[c2autoinc]

I'm not quite sure, to be honest, could be something like fuel pressure or injector but that'd just be guessing. You could try clearing the code and seeing if it comes back. If it doesn't it might have just been a hiccup

[Forged328]

I have the same code...BUMP

[Steve530]

I'm not sure if it means that the DME has reached the fuel trim limit or that the fuel trim applied does not correct the air to fuel ratio.

AFAIK, it means that you either have a vacuum leak, a bad oxygen sensor, or a problem with the MAF.

Are there other codes?

[techvet]

PO173 is fuel trim malfunction for bank 2 (cylinders 4 to 6).
Depending on the adaptation values (AKA, long term fuel trim or short term fuel trim) which can be read using a more advanced scanner and not just a code reader, this could be a rich or a lean problem. Usually, it is a lean problem, meaning an air or vacuum leak. But then, why only cyls. 4-6? So it could be either a leaky injector or a low performing injector in cyls. 4-6.
It usually does not involve the 02 sensors for otherwise it would set a fault for the 02 sensor(s).
It can also be a leaking crankcase vent valve or its various hoses.
But chances are that it is a bad airflow meter. If the engine is running roughly right now, pull the plug on the airflow meter (the sensor right after the air filter) and if it improves as it goes into default setting, then the MAF is bad.

[Steve530]

As I understand it, the car must be off when you pull the MAF, correct?

[techvet]

No, while the engine is running badly, pull the plug on the MAF and if it smoothens or
at least imnproves, VOILA!.

[Steve530]

No, while the engine is running badly, pull the plug on the MAF and if it smoothens or
at least imnproves, VOILA!.

Oh, I thought that might damage the MAF.

[thejlevie]

It usually does not involve the 02 sensors for otherwise it would set a fault for the 02 sensor(s).

I'm sorry, but that's not true. While a failed O2 sensor will set a specific DTC, an aged pre-cat O2 sensor will almost always cause a fuel trim DTC to be set well before you'll get a DTC for the sensor.

In this case an aged pre-cat O2 sensor would be my first suspicion since the fault is only affecting one bank. An intake leak or MAF problem almost always affects both banks. If the car has over 80k miles both pre-cat sensors need to be replaced as the useful life of the pre-cat sensors is not more than 100k.

[techvet]

Pulling the plug on the MAF while running will not damage it.
And if we had readings of the pre and post cat 02 sensors, it would definitely help. But we don't.
The MAF can do freaky things. And not always to both banks as one would assume.
Checking the 02 sensors or replacing them is not as easy as pulling the plug on the MAF.
And pulling the plug does not mean replacing it.

[beachwolf383]

My 97 328i just threw P0173 Fuel trim malfuction bank 2 after having the fuel show too rich and bad O2s, I replaced those then the cam shaft sensor went bad and I replaced that and now this code. could it be the MAF still? if not any clue as to what it might be?

[bmwdirtracer]

Actually, Techvet, I think there's an error in your diagnosis.

By pulling that plug, as you've stated, you put the car into fault mode, and defaulted all normal mixture calculations, not just the maf. It's running off preprogrammed maps. Symptoms become irrelevant, because fuel mixtures and airflows are computer predetermined largely from engine speed. You've defaulted more than just the maf. So the car's going to run better, even if the O2 sensor was at fault. Furthermore, you've not ruled out an intake leak, which is perfect reason for a car to run better after the maf's unplugged, since the maf's giving incorect data on airmass when plugged in, even though it's working correctly.

I very much agree with you that one needs to read adaptations, and furthermore, live data on a good scanner, to help determine leanness or richness as the cause of the code, and I further agree that individual injector flow is a highly likely, and yet little-diagnosed fault for all faults reading too rich or too lean.

But far more common, I find, are failing PRIMARY O2 sensors which don't give codes for bad O2 sensors, but only for fuel trim limits reached.

If it were my car, I'd do a smoke test to rule out intake leaks, backpressure test bank 2, watch live data from the MAF, O2 sensors, tps, temp sensors, etc.

As a customer car, it would probably save the customer a lot of testing, while providing him with normal maintenance parts only, to start by replacing the primary O2 sensors, and clearing the codes.

If the car came back, with the same bank's codes only, I'd pull the injector rail, observe flow, measure volume. Only after I had ruled out all possibilities relating to a single-bank issue would I consider replacing the MAF, since no codes had been set by it, and observance of live data had shown it to be within normal ranges.

[thejlevie]

I agree with Chris. Seeing the engine idle better with the MAF disconnected can be a red herring for certain faults.

[Placey]

check the intake boot after the mass air flow meter, i have seen many torn and cause mixture adaptation faults. Check on the bottom of the boot also, as many have overlooked checking the entire boot. (most times in the ribbed section of the boot)

Start basic before condeming a mass air flow or o2 sensors.

[thejlevie]

check the intake boot after the mass air flow meter, i have seen many torn and cause mixture adaptation faults. Check on the bottom of the boot also, as many have overlooked checking the entire boot. (most times in the ribbed section of the boot)

Start basic before condeming a mass air flow or o2 sensors.

I'd agree with that, if and only if the mileage of the O2 sensors is low (below 80k). The useful life of the pre-cat O2 sensors is not more than 100k in most cases. And they may be aged enough to cause problems at 80k. If those apply, replace the sensors along with a through search for intake leaks.

[not2quick]

Inlet booy to the throttle is a big possibility, check inside the ribs of the boot for a crack. Also you can check for an umetered air leak by letting the engine idle and spray carberator cleaned all over the intake/throttle/boot area. If the idle changes, you have a leak. Also if you can check the additive and multiplicative adaption values that would be useful, They are probebly both off, just bank 2 is bad enough to set the fault.(i thing when it is adding 8% it sets the fault)Also yes, a "lazy" o2 could also cause this, but that would normally be only if 1 banks adaptions are off. Probebly an unmetered air leak or vacuum leak.

boot, not booy

Wow, thats from 2007, OK

[Milsom1]

I've had this code come up after a few mods to my e46 323ci, I've had the car remapped and when done I was told the injectors are running rich. I'm pretty sure the code has come up because of this, possibly look into it as it could be the fuel to air ratio isn't correct.