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Audi A4 1996-2001, S4 2000-2002 (B5) P0171: Why would it run lean?


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Last Post: Jul 9, 2019 12:40 PM Last Post By: davidgad
Dyno

Posts: 13
Registered: 02/11/05
P0171: Why would it run lean?
Posted: Aug 16, 2006 9:48 AM
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I have a 2001 A4 1.8T with 105,000 miles. The check engine light recently came on. I hooked up my laptop and diagnostic software (AutoTap) and pulled the following DTC...

P0171 Fuel Trim,Bank1 System too Lean

I cleared the code, but it came back. No other codes are present, so I assume the oxygen sensor is working properly.

Observation of the parameters reveals that the long term fuel trim value gets reset to zero after clearing the DTC. Upon driving, the LT trim value slowly climbs up to about 20%, where it apparently reaches the threshold for setting the flag. The cars runs smoothly with no misfire or stumble, which leads me to believe there is not a gross problem, like a big vacuum leak.

I looked over the vacuum hoses and fitting, but nothing is obviously broken or disconnected. Hard to say if there isn't a minor crack or leak somewhere.

Anyone have experience with this? Any recommendations?

I had a similar problem with a Volvo 850. When I took it to the dealer, the service tech confidently told me it was a certain vacuum elbow. He was eventually proven correct even though he never even lifted the hood.

Are there similar trouble-makers on the Audi?

For now, I'm just going to throw a few cans of injector cleaner at it and cross my fingers.

Thanks in advance!
Dyno

Posts: 13
Registered: 02/11/05
Re: P0171: Why would it run lean?
Posted: Aug 16, 2006 1:57 PM   in response to: Dyno in response to: Dyno
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Update: Audi tech (OK, he's actually the guy behind the parts counter) told me that Audi's have very few fuel injector problems, but the service department goes through MAF's like they were popcorn. A re-focused search of this site suggests he may be on to something.

Comments?

Is disconnecting the MAF the best way to diagnose?
Dyno

Posts: 13
Registered: 02/11/05
Re: P0171: Why would it run lean?
Posted: Aug 23, 2006 11:07 AM   in response to: Dyno in response to: Dyno
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It was the MAF sensor(or AMM, if you prefer). Long-term fuel trim values floated down nicely from 25% to 0% after replacement.

As an aside, this was the first time I had the airbox lid completely removed. I was not aware of the pre-filter screen that catches the rocks and dirt before air enters the air box. I'm surprised that any air got through there at all! It was easy to remove and clean. I bet that was good for 20 hp!
play

Posts: 13
Registered: 12/04/17
Re: P0171: Why would it run lean?
Posted: Jun 22, 2018 6:18 AM   in response to: Dyno in response to: Dyno
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davidgad

Posts: 8
Registered: 07/09/19
Re: P0171: Why would it run lean?
Posted: Jul 9, 2019 12:40 PM   in response to: Dyno in response to: Dyno
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