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  #1 (permalink)  
Old 07-30-2008, 08:12 AM
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Exclamation Sentra P0171 system too lean (bank 1)

the check engine light has been on for quite some time. I've been told this means that there is too much air in the air/fuel combination. It has been suggested that I use a fuel injecter cleaner, did that no change. Also that I cleaned the MAF, no change, and that I make sure the gas cap is on tight..did that no change. I got a diagnostic at autozone and got the code p0171 system too lean bank 1. My car started fine, just occasionally stalled.Yesterday i was driving and the car just stopped, like the car wasnt getting any fuel..just had oil change and had a full tank of gas.
i had to put it in neutral and pulled over. The car wouldnt start..no clicking, just like no fuel was getting to the engine. People have told me it could be anything from replacing the MAF, the o2 sensers,cat. converter,fuel injecter, fuel pump.I went out this morning and started my car..seemed fine but I wont drive it untill i take it in. I just want a few a suggestion about what it could be.Thanks
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Old 07-30-2008, 08:19 AM
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People have told me it could be anything from replacing the MAF
MAF is most likely the cause of the problem.
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Old 07-30-2008, 09:08 AM
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what is the connection between the fuel getting to the engine and the MAF? I want to have some idea of how or why..thanks for posting.
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Old 07-30-2008, 12:51 PM
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Originally Posted by NissanTech View Post
MAF is most likely the cause of the problem.
+1 on the maf
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Old 07-31-2008, 08:13 AM
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When you press the throttle pedal, the throttle plates open and that lets air rush into the engine. The amount of air going into the engine is measured by the MAF. The MAF sends that information to the ECM and depending of the amount of air going into the engine (an other inputs, like engine temperature, speed.. etc) the ECM decides how much fuel is send to the engine. The ECM controls the time amount of fuel by lengthen or shorten the injectors opening time.

I hope that helps.
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Old 03-30-2010, 10:24 AM
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After reading all of these, and other posts of a similar nature, I am wondering why you are pointing to the MAF when it's only a P0171 code. I've been getting P0171 ONLY, never P0174. The MAF is common to both banks, yes? The only things I don't see as being common to both banks are the injectors and a possible intake/exhaust leak.

Is it possible to get P0171 without P0174 ever appearing, and have a defective MAF?
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Old 03-30-2010, 04:45 PM
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You refer to both banks but there is only one bank.
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Old 03-30-2010, 05:22 PM
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Originally Posted by tech22 View Post
You refer to both banks but there is only one bank.
2000 Nissan Sentra GXE with the QG18DE engine. 102,000 or so miles.

I have four O2 sensors, two per bank, front and rear (before/after the catalytic converter).

As I understand it from the service manual, the outer two cylinders are bank 1 and the inner two are bank 2.

Here's my story:

About a year ago, I was getting an intermittent P0171 code (Bank 1 System Too Lean), which would clear and re-appear. It finally stayed on. Seems to come on as I'm accelerating on the highway through about 2,500 RPM and 40-50 MPH.

It reliably re-appears after I clear the codes. Last week, the engine stated running very rough, and I was now getting codes

P0134, P0154 (Bank 1/2 front O2 "high voltage").

The PCM, through the OBD-II port, was reporting that both sensors were off-scale high, reading a steady +1.275 V. Direct voltage measurements using probes at the PCM terminals themselves were showing slowly fluctuating NEGATIVE voltages. The hotter the engine was, the more negative the voltage from the sensor. And it was a very slow change. Totally abnormal, it seemed. The sensors should be putting out between 0-1 V. In any case, the measurements I was taking off of the sensors didn't match with what the PCM was reporting.

So, I didn't know whether I had a ground problem, a power short, bad sensors, or a bad PCM.

I did a continuity check of the harness according to section EC, page 256. Harness OK.

I did the diagnostics for the O2 heater circuits as well, with normal OK results.

After consulting with a wise and seasoned electrical engineer (my father-in-law), I ended up rigging a test harness using my laboratory power supply and a resistor divider network to feed a known voltage into the PCM on pins 62 and 94 (the bank 1/2 front O2 sensor signal pins), and see what came out the OBD-II port.



Turns out the PCM was reporting the exactly correct voltages to within 1 mV, which was a huge relief. My theory is that the negative voltages from the sensors were confusing the analog-to-digital conversion in the PCM and it was freaking out and interpreting the negative voltage as an off-scale high value.

So, having eliminated the PCM and the harness, the sensors were the only things left as suspect. I replaced both front O2 sensors with Bosch exact-fit OEM replacement parts, and reset the PCM by leaving the battery disconnected, then performing the "P100" procedure by disconnecting/reconnecting the MAF.

The car ran like crap, and I was doing circuits around the house within a 5-block radius or so in case I got stranded. After a few miles, it started running very well, so I ventured out onto the highway and did a few high-speed runs back and forth. The car was running GREAT! Smoothest idle I've had in a long time, good acceleration, good power, and the sensors were putting out a nice 0-1 V oscillating signal.

I thought I had everything fixed.

Today, on the way to work, I got the dreaded P0171 DTC pending-only, meaning no SES light. On the way home, it became a full-fledged DTC and I dumped out the freeze frame as follows:



Looks like the long-term fuel trim is way too high. Perhaps it is the MAF? Then why only a P0171 and not a P0174 as well?

My guess is that the O2 sensors finally went out on me, and are unrelated to the original P0171 problem. I spent the weekend over-analyzing O2 sensors that finally went bad while not really addressing the original problem.

Should I be looking for an intake or exhaust leak, does the PCM need to be "reprogrammed", or is this really the sign of a defective MAF sensor?

I don't understand what tech22 says about "only one bank". Everything I'm reading and seeing indicate that I have two banks.

The Sentra is running better than it has been since I can remember, but I'm still unable to get my emissions tested until I fix this one.

Let me add that this site has been a very, very useful thing to me and I'm extremely grateful for it and all of the people who contribute!

Last edited by pzaremba; 03-30-2010 at 11:42 PM.
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Old 04-01-2010, 09:06 AM
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Followup:

Should have listened to tech22. No obvious vacuum leaks. Finally decided to take a stab and replace MAF sensor due to slightly unusual RPM vs. MAF sensor output signals, as viewed on a scatter plot. I was out of options.

No more P0171, but during my "learning" drive, friggin' code P1491 shows up as a "1st trip" (no SES light)!!!!

I'm sitting there at the top of the ramp going, "oh, WTF is that????"

So I get it home, all of the sensors are reporting ready, SES light is off, and I hold my breath and cross my fingers that I can get the emissions test today.

Well, I babied the car all of the way there and PASSED! When I got to work, I checked the codes with my scanner and the P1491 had been cleared, so it must have been a transient thing.

I vaguely remember my wife having this code couple of years ago, and the repair guy said she needed a new catalytic converter. BS.

Thanks to this site, I know that it's the vacuum cut valve bypass valve, which I now have a year and a half to diagnose and fix if necessary.

THANKS AGAIN to this site, the folks here, and a mea culpa for not believing that my MAF needed replacement.

I'm still wondering about the "1 bank or 2 banks" question.

Last edited by pzaremba; 04-01-2010 at 09:09 AM.
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Old 04-03-2010, 05:34 PM
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Great job on that one.From your photo it would appear you really ment to get to the bottom of the problem.These nissans like to play tricks and the service manual tends to condem the ecms alot.Your frist code went into history and remained there.Everytime you started the vehicle the ecm looked for that code saw it and would repeat that process three times or three (complete) drive cycles.Clear as you might it was a hard code that stayed in the ecm's history untill you corrected that problem and it didn't see it again.Go to your service manual under chapter EC and preform the idle air/volume air proceedure and she should run great.Nissan grouped the two banks for both 4 and 6 cylinders with there codes.

Last edited by tech22; 04-03-2010 at 05:59 PM.
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Old 04-04-2010, 09:35 AM
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Originally Posted by tech22 View Post
Go to your service manual under chapter EC and preform the idle air/volume air proceedure and she should run great. Nissan grouped the two banks for both 4 and 6 cylinders with there codes.
Thank you very much for your compliment.

I did the idle air volume learning procedure last night, and at first observation, it seems to really have smoothed things out. I didn't get a chance to really put the car through its paces, but am looking forward to doing that and getting a look at the fuel trims.

Thanks a lot for your advice.

I guess I still don't understand...so the two banks on my engine are grouped? Does this mean I would never get a P0174 code? I thought that I saw others here with the QG18DE engine getting bank 2 only codes.

Thanks again!

-paul
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Old 04-05-2010, 04:41 PM
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I'am very sorry and stand corrected. Form some odd reason i was thinking of a different model year.Hope i didn't get you to messed up.Once again sorry for the mix-up.
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