Having this same P0171 error code on a 2008 Eos 2.0T 74K miles that I just bought. Of course, the check engine light came on first time on the way home from purchasing. Grrrrr! Have read both links above provided by silvershadow (thank you). In the week since owning, I have cleared the code, driven the car around town for about 90 miles, and had the light come on and off twice without me clearing the codes, which I thought was odd. Its back on presently after about 25 miles. No discernible difference in idling or engine performance to m, seems fine except for the check engine light, its the only code. Have an appointment to have it checked tomorrow but was wondering if the mileage distance driven and the off/on are clues to what may be the cause. To be sure, my basic OBDII reader says its "system too lean, bank 1". And also, it has not been highway driven or up over 60 mph during this week, just around town and idling. TIA.
The codes, check lights and warnings are there for you to do something or investigate the problem further before engine damage, rather than wish it goes away on its own. But warnings can be caused by bad sensors, connectors and wiring.
Just on the error code alone if the sensor is telling the truth, there's a bunch of things to check if you are up to it or else leave it to the workshop techs. You can't easily judge faults in a closed loop Efi engine system just by sound and feel. If the mixture stays too lean for long and burns a valve, you will get more feedback and need deep pockets for the repair.
I get it that a warning light is an alert to have it checked, and I have made the appt, but if the code doesnt reappear for 90 miles, and if the light was back on then went off on its own, is that telling in any way, that the issue is intermittent?
Your generic code reader will not tell you much. A diagnostic tool that can talk to VW group cars (see link in my signature) will tell you whether the fault is intermittent or static, along with how many times it has been triggered since it was originally logged.
If you read the link I posted there isn't one simple fix and several possibilities. Most basic diagnostics tools will spit out a stored fault code, but tools like OBD Eleven or vcds can also 'measure' components in the engine system that are sensored. Used correctly by an experienced technician to get more information, they avoid the expensive and illogical 'scatter gun' approach of replacing everything in sight, which is a common mindset of those who think they know more than garage technicians.
Definitely worth a smoke test; I think I'd do that first, knowing that the PCV is a weak spot on these cars. Then secondly, looking at injector(s) and fuel pressure stuff (I think the scan tool has a built in program to test this?)
I do appreciate the continued support on this. It will help me in discussion with the mechanic tomorrow which is the first time I've used this mechanic. So asking what scan tool they are using will help me know their level of sophistication as well. I have read ALL links and posts connected to this issue both here and at vwvortex.
It's not just what scan tool they use, but whether they are competent at using it for doing more than 'press for scan' and read out fault codes. When there are possible multiple fault scenarios to check and diagnose you need competent humans with a logical approach, brains and training in the driving seat. You can't captain a boat in all weathers and make the right decisions using Google. But Google might be automatically driving boats in future and some may sink or collide.