Error Code "P2098 Post Catalyst Fuel Trim Bank 2 System Too Lean

Thank you guys in advance for your time and expertise.

History:
The vehicle in question is a 2002 BMW X5 3.0i with 71k. In the past 6 months, I had a throttle housing replaced with the original BMW throttle. During the reprogramming of the throttle at the dealership, an error code for the rear oxygen sensors were going off, but they were reset and believed to have been an unrelated glitch. Shortly after this repair, I was struck from behind and the left exhaust system and bumper were replaced.

Soon after, the rear O2 sensor error message returned. For this reason, I replaced the two rear O2 sensors with Bosch and not the original BMW O2 sensors. During the replacement of the O2 Sensors, one of the threads where stripped while prying the old one out. However, I was able to re-thread the hole opening in order to insert the new O2 sensor, but there may be a leak at this particular junction.

Two weeks ago, I noticed that the temperature gauge was defective and I replaced the engine thermostat and the issue was resolved.

Within a couple of days, my check engine light was present and the error code "P2098 Post Catalyst Fuel Trim Bank 2 System Too Lean."

Car Symptoms and most recent error code P2098:
Despite the P2098 code, there are no noticeable inefficiencies while driving. In addition, my Miles Per Gallon ("MPG") gauge displays inconsistent readings. For example, when I'm driving off the highway and I release the accelerator and apply the breaks, simultaneously the MPG gauge drops from 40MPG reading to 15 MPG reading and then returns to 40 MPG. Similarly, this occurs immediately after an aggressive acceleration cycle followed by a lack of acceleration with or without breaking to a complete stop.

Discussion:
What could all this mean? Could the P2098 code and MPG reading be related? Could this also reflect to much air, or not enough fuel? Could the P2098 code reflect a vacuum leak, fuel problem such as pump or filter? How about a defective Mass Air Flow Meter? But if were the case, wouldn't this also trigger an error code from the front O2 Sensors?

Thank you in advance for your input.

Here are some possible explanations for the too lean error code.

A friend of mine with a diagnostic computer, similar to the ones at the dealership ran a diagnostic. One issue that he uncovered was that the O2 sensor's built in heater was not turning off even when the desired temperature was reached.

Subsequently, I shared this information with my buddies at the BMW dealership and their preliminary prognosis is a vacuum leak. Either to much air is going into the engine when its not suppose to or a leak by the oil separator, which will trigger a too lean error code.

Now, I must locate where there is evidence of a leak.

Originally Posted by Cito54

Thank you guys in advance for your time and expertise.

History:
The vehicle in question is a 2002 BMW X5 3.0i with 71k. In the past 6 months, I had a throttle housing replaced with the original BMW throttle. During the reprogramming of the throttle at the dealership, an error code for the rear oxygen sensors were going off, but they were reset and believed to have been an unrelated glitch. Shortly after this repair, I was struck from behind and the left exhaust system and bumper were replaced.

Soon after, the rear O2 sensor error message returned. For this reason, I replaced the two rear O2 sensors with Bosch and not the original BMW O2 sensors. During the replacement of the O2 Sensors, one of the threads where stripped while prying the old one out. However, I was able to re-thread the hole opening in order to insert the new O2 sensor, but there may be a leak at this particular junction.

Two weeks ago, I noticed that the temperature gauge was defective and I replaced the engine thermostat and the issue was resolved.

Within a couple of days, my check engine light was present and the error code "P2098 Post Catalyst Fuel Trim Bank 2 System Too Lean."

Car Symptoms and most recent error code P2098:
Despite the P2098 code, there are no noticeable inefficiencies while driving. In addition, my Miles Per Gallon ("MPG") gauge displays inconsistent readings. For example, when I'm driving off the highway and I release the accelerator and apply the breaks, simultaneously the MPG gauge drops from 40MPG reading to 15 MPG reading and then returns to 40 MPG. Similarly, this occurs immediately after an aggressive acceleration cycle followed by a lack of acceleration with or without breaking to a complete stop.

Discussion:
What could all this mean? Could the P2098 code and MPG reading be related? Could this also reflect to much air, or not enough fuel? Could the P2098 code reflect a vacuum leak, fuel problem such as pump or filter? How about a defective Mass Air Flow Meter? But if were the case, wouldn't this also trigger an error code from the front O2 Sensors?

Thank you in advance for your input.

First and foremost have you ensured the fuel cap is tight and sealing properly? Lack of tightening / sealing of the gas cap can result in the check engine light illuminating. So much so there was a special placard in the owners packet calling attention to it.

With that said my 2000 X5 4.4i had P1090 (Pre-Catalyst Fuel Trim Too Lean Bank 1 ) set and it turned out to be the Mass Air Sensor. From my research the MAS is one of the most likely reasons this particular code is set. Since yours is not the exact same code and you've had problems with the O2 sensors I can't say for certain if it really is the MAS. But I wouldn't be surprised to hear the MAS ends up being the problem. The mileage you have on your X5 is approaching the lifetime of the MAS (approximately 80K I'm told).

The good news is the MAS is easy to replace (at least on the 4.4). The bad news is it can be pricey (if it turns out it is not the problem). Mine cost $300.

Also how are you getting 40MPG out of your X5? Does the 3.0 really get that good of fuel mileage?

Originally Posted by sunny5280

First and foremost have you ensured the fuel cap is tight and sealing properly? Lack of tightening / sealing of the gas cap can result in the check engine light illuminating. So much so there was a special placard in the owners packet calling attention to it.

That would set a different code. Since he has the code results, it has nothing to do with the fuel cap.


Typically, I would expect the pre-cat to also give a "too lean" code if it was a vacuum leak.

Personally I'll bet if you replaced the Bosch O2 sensors with OE sensors, the error would go away. I have had problems with Bosch O2 Aftermarket sensors on non-BMW vehicles.

Over time, the Mass Airflow sensor (MAF) will become fouled with gunk- oil from aftermarket filters, exhaust particles, stuff- which builds up a slightl insulative layer. This has the effect of telling the DME there is actually LESS air flow than there really is. The DME then gets 'stuck' between reconciling the AFR it is calculating from the MAF value and the AFR it is calculating from the O2 sensor reading... the difference between this is a number called the long term fuel trim LTFT. Once this gets past a certain value, you get a code.



So, FIRST THING, Take the MAF out and clean it with MAF cleaner. If this helps, consider a new maf. The cleaner can take off some of the gunk, but there is almost always residual.

Originally Posted by ard

Over time, the Mass Airflow sensor (MAF) will become fouled with gunk- oil from aftermarket filters, exhaust particles, stuff- which builds up a slightl insulative layer. This has the effect of telling the DME there is actually LESS air flow than there really is. The DME then gets 'stuck' between reconciling the AFR it is calculating from the MAF value and the AFR it is calculating from the O2 sensor reading... the difference between this is a number called the long term fuel trim LTFT. Once this gets past a certain value, you get a code.



So, FIRST THING, Take the MAF out and clean it with MAF cleaner. If this helps, consider a new maf. The cleaner can take off some of the gunk, but there is almost always residual.

What you say makes sense to me except for one thing... why is the code for a "post catalyst" O2 sensor? Typically, the post catalyst O2 sensor is only used for monitoring the catalyst efficiency, and not for fuel trim or other operational adjustments. My understanding is that it is only the pre catalyst sensor that is used for fuel trim and adjustments. Yet the message refers to fuel trim and post catalyst in the same sentence. Is BMW doing something unusual here, and if so what.

[Correcting an earlier comment in this message, the P2xxx codes apparently are generic, not manufacturer-specific, according to this site http://www.gendan.co.uk/article_4.html ]

Originally Posted by Penguin

What you say makes sense to me except for one thing... why is the code for a "post catalyst" O2 sensor? Typically, the post catalyst O2 sensor is only used for monitoring the catalyst efficiency, and not for fuel trim or other operational adjustments. My understanding is that it is only the pre catalyst sensor that is used for fuel trim and adjustments. Yet the message refers to fuel trim and post catalyst in the same sentence. Is BMW doing something unusual here, and if so what.

[Correcting an earlier comment in this message, the P2xxx codes apparently are generic, not manufacturer-specific, according to this site http://www.gendan.co.uk/article_4.html ]

I totally agree. The post cat sensor is not used for trim nor AFR...hence the reference to 'lean' makes no sense (to me) if it is indeed a 'post cat' measurement... My advice is essentially ignoring the 'post cat' part. ( Seemed reasonable since cleaning a MAF is $10 for the cleaner and 10 minutes.)

If one had a Peake reader, you might not have to deal with the generic codes and you have the BMW-specific codes (ie P2098 may resolve to something specific in the BMW lexicon that is non-sensical in the generic list)

A

Originally Posted by ard

I totally agree. The post cat sensor is not used for trim nor AFR...hence the reference to 'lean' makes no sense (to me) if it is indeed a 'post cat' measurement... My advice is essentially ignoring the 'post cat' part. ( Seemed reasonable since cleaning a MAF is $10 for the cleaner and 10 minutes.)
A

I agree with cleaning the MAF, but if one ignores the "post cat," then I would also check for a vacuum leak. My 2004 Z4 developed a vacuum leak due to the rubber intake boot between the air cleaner and the throttle body/intake manifold. The boot has a smaller "pleated" flexible air tube coming off of it and it had cracked at the bottom of some of the pleats causing a small air leak. But I got the typical "too lean" code P0174.

Summary:
I took my car to a repair shop that specializes in foreign cars because service engine soon light was on - Lean code (P2089), and MPG gauge sweeps back to 14 MPG from 40 + after applying brakes.

Regarding the latter, I was told that my MPG is responding similarly to other older X5 models. Thus, it seems to be working properly and nothing else was done.

Regarding the lean code, my father, who is a diesel mechanic for 40 years, installed the new 02 sensors incorrectly. It was such a simple fix, but I had to shell out $326.42 for three hours of labor.

Below is the work description from the mechanic:
Service engine soon light is on. Scan test for codes; P2089 post catalyst fuel trim too lean bank 2, 190 02 sensor 1 after cat heating, 222 02 control inactive after time limit, 226 fuel trim control after catalyst bank 2, clear codes.

Road test, code 226 returns.

Smoke test intake, no leaks. Check for tech bulletins, 12/07/04 for 02 sensor re-program.

Road test multiple times, code 226 returns by 2nd trip. Monitor data stream on road, perform individual test on 02 sensors.

Trace wiring for sensors, plugs were reversed for down stream sensors, correct.

I leaned a lessen here. Sometimes it best to bring your car directly to a BMW mechanic in the first place. I was penny wise, pound foolish.