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SSEi95 10-02-2003 05:47 PM

DTC P0171 / Fuel Trim Lean...
More of a story than a question...

I get this SES light that apears about once every 2 months. I finally got a scanner on my car right after it showed so I can pul it. What I got was DTC P0171 / Fuel Trim Lean.

So I went into my magic factory service manuals and here are a few of the "preliminary checks":
  • Exhaust system for corrosion, loose or missing hardware (I have a high flow 3" exhaust)
  • Heatedoxygen sensoris installed securily and the pigtail harness is not contacting exhaust manifold or ignition wires (new O2 a while back, problem started a ways after it was installed)
  • Vacum hoses for splits, kinks and proper connections (rebuilt S/C and had that all ripped apart about a year ago)
  • IAC; if a high or unsteady idle is being experienced, scan idle air control with a Tech 1. If idle air control displays near 0 counts, use chart c-2b, idle air control valve check. (my idle is around 950 - 1000 and does fluctuate some, but I also have 2.5 pulley, MSD packs, better wires and plugs, no MAF screen)
  • Crankcase ventilation valve, spring and O-ring for proper installation. Refer to section "6e3-c13".
  • Throttle body inlet screen for damage and/or foreign objects (screen removed :wink: )
  • Fuel for excessive water, alchol, or other contaminent (I filled up with half a tank of 87 so I did the other half with 93 and added octane booster)
  • PCM and sensor grounds are clean, tight and in their proper locations

So is it even worth me tracking this down? This is only the first level of troubleshooting. There is 2 more pages of trouble shooting after this. :? Since it only happens rarely I won't loose sleep over it, but it's always better SES light free! :D

A Bonnie Canuck 10-02-2003 06:08 PM

The dreaded P0171!!!
As a mechanic I can tell you with no other codes and no real driveability problems and some modifications to the air flow, you can bang your head against the wall trying to figure this one out. Is it really a problem? I would watch the O2 sensor switching with the diagnostic display. If it is switching between rich and lean with good frequency, the chances are the O2 sensor is good. It may just be that under some circumstances with the increased air flow you may get a slightly lean condition at times. If it is only once every 2 months (I am envious!!!), I wouldn't worry too too much about it. Especially if it is running fine.

SSEi95 10-02-2003 06:52 PM

There are no other codes and the O2 is switching every .5 - 2 seconds between lean and rich. The only time I ever see this light come on is when I hit the gas about half throttle or better, and that is only every 1.5 months or more.

I am thinking way too much work for this one little code. I'll will probably just let it go but scan codes as they happen to see if it gets worse or if I get any others. Thanx for the input.

fuddyduddy121 10-03-2003 01:04 AM

If your mass air flow sensor is not representative of the air flow going into the engine, I would think that it would cause this code. You may have a lot more air going into the engine than the MAF sensor is reading because you have no MAF screen.

A Bonnie Canuck 10-03-2003 05:23 PM

The mass air flow sensor are the heated wire type. There are 2 heated wires that require a set amount of current to allow them to stay at a set temperature. The air rushing past them cools one down therefore requiring a greater amount of current to heat it up to the set temperature. This current increase is measured by the PCM and compared to a table. This table will then, along with Intake Air Temperature readings, give the air flow volume (proportional to engine load). So even though you have more air flow going into your engine, the MAF will sense the flow volume no matter what it is. If you had a problem with your MAF (I have twice) you will definitely have driveability problems and not necessarily a code set either :(

brminder 10-03-2003 05:53 PM

I'd check some more for vacuum leaks.
The Darwin-award way would be to buy a can of starter fluid and whilst the engine is idling spray it around the intake and TB and hoses to see if the idle changes.
Seems to me someone suggested getting somthing you can spray that isn't flammable and would make the idle drop, but I can't remember what it was.
Starter fluid would be more exciting.

TrueWildMan 10-03-2003 06:19 PM


Originally Posted by SSEi95
I hit the gas about half throttle or better, and that is only every 1.5 months or more.



SSEi95 10-04-2003 12:52 AM


Originally Posted by TrueWildMan

Originally Posted by SSEi95
I hit the gas about half throttle or better, and that is only every 1.5 months or more.



oh, I was meaning the error every 1.5 months. The foot thru the floor is a more than daily occurance. :lol: :twisted:

SSEi95 09-09-2004 08:43 AM


This has come back more prevelant since I have installed my new CAI. However, there is a scenario for it now. Driving down the road it does not come on anymore. The only times it has come on is when I idle for a minute or two (whether at a stoplight or just in the driveway). I have also noticed that the idle will become noticebly (barely) more rough right before it throws the SES light.

Is there something inpaticular I can check to make sure everything is operation correctly? Could there be an issue with the injectors, lack of a working cat, or ignition system? I do have new MSD wires at home to install this weekend and Denso plugs are on their way. Not sure if this will help alot ubt I'm due for some maintaince anyways.

coliphage 10-04-2005 02:08 AM

Have you solved the problem?

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