It was suggested to me that O2 sensor could be guilty party. Vacuum lines check out ok. Engine is OBD II ..... The Bentley's explanations are kinda vague. It mentions that it's too lean. Anybody know ? Thanks.....
16555 P0171 Fuel Trim,Bank1 System too Lean
Your ECM has a memory look up table that tells how long the fuel injectors SHOULD be open, to give correct air-fuel. The Oxygen sensor monitors the exhaust to tell how incorrect the table is. The table is programmed when the car is new and everything works fine. As the car gets old you may have wear & tear and things vacuum leaks, etc. This causes the table to be off. THe O2 sensor tells the ECM how to tweak the fuel injectors back to give the correct air-fuel. Basically, the car learns how to correct the table for the current running condition of the engine.
In your case the ECM (with O2 sensor) has determined that it has used up its entire fuel injector tweak-tuning range for correct air-fuel. You're running too lean and the car can't add enough fuel to bring the air-fuel back into range. Your either getting too much unmeasured air (air coming in besides through the MAF sensor) or you're not getting enough fuel pressure. That, or the MAF is measuring the incoming air incorrectly.
Your problem is one of either:
A) low fuel pressure or fuel delivery rate
-bad (weak) fuel pump
-clogged fuel fiter
-failed fuel pressure regulator
-clogged/failed fuel injectors (like multiple injectors, unlikely)
B) vacuum leak
- something severe like unplugged hose or torn open hose
C) Bad MAF (mass air flow) sensor.
The MAF is probably the most expensive problem, followed by a new fuel pump. I suggest you get a fuel pressure tester and test the pressure and delivery rate. That will tell you if it's the pump, filter or regulator. Perhaps put on a new filter anyways, considering they're cheap. If it's not fuel delivery or pressure then it could be the MAF.
First thing is to check for any vacuum leaks.
Anytime you make a change, unplug the battery -ve cable for a couple minutes. That will clear the engine light and any check engine codes (DTC's). More importantly, it makes the ECM forget any learned settings (tweaking/tuning of fuel injectors) and now it will begin to learn and to re-tweak the fuel injectors from the original factory memory look-up table. Don't forget that this "tweaking" and "learning" doesn't begin to occur until the car has warmed up enough for the oxygen sensor to get hot. Therefore, go for a short drive and get everything warmed up to see if the check engine light comes back on. If it stays off then the problem may be fixed.
I would suspect the ECM determined this problem by looking at the O2 sensor output... therefore it's not the problem part. Also, if the O2 sensor were dead then you would have a DTC specifically telling you it's dead. I wouldn't change it just yet, unless it has 100k miles on it.
Modified by bearing01 at 4:29 PM 5-1-2006
I have the same code right now. After doing some research, logging with vag-com, and looking around the engine... i am leaning towards a bad MAF.
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i have the same code but i run lean at idle (add.) my idle is crap and need to figure this out. i have new maf and 02sensors. i have checked for vacuum leaks, smoked tested the engine, found no leaks. it feels like a vacuum leak, have a new FPR still not fixed. this problem only occured after install of the stg3+. must have f'ed something up. any help?????
I had the same code on my 02 passat. MAF o2 and vacuum lines all ok. I even sprayed half a can of starter fluid around my engine trying to find the leak. mine was not vacuum it was pressure leaking from the oil filler cap. use a little soapy water around the edge to check it while its running. hope this helps
Wow, sounds like a common problem with these older cars, my mother in law's jetta is running rough and wanting to stall at idle. i will check a few things and scan it. It also has that tell-tale smell of rotten eggs out of the exhaust
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welp mine was none of those, turned out to be a leak in the EVAP system and a bad purge valve causing lean fuel trim at idle