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Code P0171 system lean

06-27-2010, 01:09 PM
Getting the above code on my scanner. I have a Haynes manual but it only tells me the system is lean, and I can't find what to check.

The car is a 2001 Regal LS with 194,000 miles on it. I've only had it for a month so I'm not real familiar with the car.

When I first got the car I started to put a fresh set of plugs in it, but I've only changes the front bank so far, and I'm trying to work myself up to fighting the rear plugs. I can reach the rear wires but I can't pull hard enough to get the wires off. The first time the code set was as I was getting ready to go on a trip. I filled up with a different brand of gas than I had been using, and then drove to the car wash to wash the car. I didn't wash the motor, and when I started the car, it felt a little rough, and the light came on. I checked the code and came up with the one above. Other than feeling a bit rough when hot the car ran fine. No pinging or loss of power is apparent.

After a week of using the car the light went out and stayed off for a week so the problem seems intermittent. Two nights ago after the car was started (once again it was warm) and I headed up the road, the light came on again. The car was running fine when the light came on.

Anyone know what I could check out? Thanks.

06-27-2010, 02:50 PM
DTC P0171 indicates the system is in a lean trim mode. Ideal fuel trim = zero. Greater than zero indicates the PCM is adding fuel to compensate for the lean condition. If fuel trim is less than zero the PCM is reducing the amount of fuel to compensate for a rich condition.

Here is the course of action per the GM service manual:

Check all vacuum hoses at intake system.
PCV Valve
Possible throttle body blockage (clean input screen and body)
IAC (idle air controller valve)
Check fuel injectors (bad o-ring seal)
MAF sensor
EGR valve
Check 02 sensor (Front)
Contaminated fuel

Best way to diagnose it is with a odb-ii scan tool capable of checking short and long term fuel trims.

06-27-2010, 02:52 PM
Usually a P0171 can be a faulty oxygen sensor, but first I would check for a few things :missing, loose, or leaking exhaust components, forward of the O2 sensor, vacuum leaks at the throttle body or injector o-rings for leaks, air induction system and air intake ducts for leaks, check the secondary air injection (AIR) system for leaks, improper air delivery, shut off valve not closing, or crankcase ventilation system for leaks. In order to check the O2 sensor, you need either a live data scan tool or a multimeter to watch the O2 sensor readings. Readings should fluctuate between about 200mv and 800mv. The reading should fluctuate back and forth constantly. If the fluctuation seems slow and sticks, replace the O2 Sensor. Like I said, the O2 sensor will probably fix it with the mileage on the vehicle, but I'd do a quick look over of the places mentioned above to ensure there are no vacuum or exhaust leaks. Hope this helps!

06-28-2010, 11:11 AM
Thanks for the help. I'll get busy checking the car over. I have a scanner that I just got to help me so once I figure out all that it can do I'll run a check.

06-28-2010, 02:51 PM
You would think the O2 sensor would be the most likely suspect for P0171 but in reality it rarely causes the problem. Vacuum leaks & inaccurate MAF sensors are much more likely to be the problem.

Do a scan of the MAF sensor & both the long & short term fuel trims. Get all three readings at both idle & 2,500 rpm. Let us know what the readings are & we'll continue.

06-28-2010, 05:01 PM
Correct me if I'm wrong, but wouldnt a MAF problem cause a lean problem on both banks? Usually when the MAF is suspect, it will throw a P0171 and a P0174 for both banks lean.

06-28-2010, 05:26 PM
P0174 isn't a valid code on the 3800. Both banks use the same O2 sensor & I'm guessing the same fuel trims.

06-28-2010, 07:27 PM
Oh ok. Gotcha. I did not know that

06-29-2010, 11:38 AM
Here's the info from the scanner.


3.46-3.57 MAF GR/SE

20.3 ST FTRM %

16.4 LT FTRM %

.15 O2 Sensor

2500 RPMS

MAF GR/SE 12.39 to 12.75
ST FTRM % Varied from 12.39 to 0 to 1 and all other kinds of #'s
FT FTRM % 16.4
O2 Varied from .130 to .800

Even though you didn't ask for the O2 sensor info I threw it in anyway. Also noticed the idle varied quite a bit on the scanner. On the scan I did to get these numbers it varied from 909 to 935 RPMS. On the scan I did before this one it varied from 860 to 922 RPMS.

Haven't had the chance to check over the other things listed in the above posts yet, and won't get to it until this weekend, but I will check them out. I have a new PVC valve that I haven't put in yet, and will clean up the intake system. A vacuum leak also my make sense because of the wacky idle.

06-29-2010, 03:14 PM
Vacuum is highest at idle & so are your fuel trims. This leads me to believe that you do have a vacuum leak. I'm going to go scan my Bonneville & see what I get for the MAF sensor readings. I'll report back shortly.

06-29-2010, 04:55 PM
My readings also fluctuate. I'm posting what seemed to be the average.
Here's what I got idling (about 825 rpm) in park:
MAF - 4.7
STFT - 1.0
LTFT - 7.8

At 2,500 rpm:
MAF - 14.8
STFT - 0
LTFT - 3.9

I then tried an experiment & created a vacuum leak by removing the vacuum line that ends up going to the fuel pressure regulator.

MAF - 3.3
STFT - 26.5 (I thought the max was around 20. This may be an anomaly with my scanner.)
LTFT - 16.4

2,500 RPM
MAF - 13
STFT - 0
LTFT - 10

06-30-2010, 05:30 PM
Got to start checking things out and found an uncapped nipple on the intake manifold side on the fitting that holds the large vacuum hose that runs to the brake booster. Plugged it up and got new scanner readings:


MAF 4.14
ST Trim 0 to 2.3
LT Trim minus 1.0

Idle speed now varies roughly + or - 25 Rpms


MAF 14.45
ST Trim 0 to 2.3
LT trim minus 1.0

Now idles much smoother and has a lot more power. I still plan to do a good cleaning on the intake parts this weekend, including the IAC because that idle seem a little low to me.

One last question. While looking over the vacuum lines and electrical connections I found an unplugged male connector with two pink wires on it. Is this suppose to plug into something, or is a test connector for the fuel pump or something else?

Thanks a lot to you folks for helping me me out with this.

07-01-2010, 03:47 PM
I'm sure it goes to something but im not real familiar with that engine but I dont think there is a test connector for the fuel pump or anything.

07-01-2010, 05:47 PM
One last question. While looking over the vacuum lines and electrical connections I found an unplugged male connector with two pink wires on it. Is this suppose to plug into something, or is a test connector for the fuel pump or something else?

That is the fuel pump test connector you see. I have a fuel pump test connector on my '99 Regal LS and '01 Regal GS (sold). It is located under the main engine compartment fuse box on the right side of engine compartment. If you need a pic showing it and the location I'll be glad to post one. Mainly for the disbelievers. :rolleyes:

07-02-2010, 06:12 PM
Thanks but I don't need the picture. Car is running really nice again. With the weekend coming I'll get to clean the intake system up and maybe change the rear plugs.

My weekend just got a lot busier because I just picked up a really nice 95 Mercedes-Benz C 280 for my wife as she just retired from a state job.

10-17-2010, 04:39 PM
I'm trying to fix the code P0171 on the Ford Contour 99 2.0 LX myself. The car is running perfectly. It has new O2 sensors, fuel filter, sparks, spark plugs, full clean injectors. I don't see any vacuum leaks or loose/cracked hoses.

I need some tips on to how to do a better tests on vacuum leak, PCV valve and ERG valve.

The car has been running for 2 years with P0171 on it and doesn't have performance problems.

Thanks all.

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