I have a 2001 A4 Quatro (AWM Engine Code), about to hit 90k miles.

The CEL was on for the last few months due to a bad 02 sensor (pre-cat), which I recently replaced. I saw some broken wires on the old O2 sensor when I was removing it, so I'm satisfied that it did need to be replaced.

Now I'm getting a P0171, System Lean Bank 1. I read up on this forum for possible fixes, and I've since cleaned my MAF, put in new plugs, replaced a frayed vacum line (the little one at the front of the manifold), and put in a new air filter.

Each time, the CEL comes back on after about 15 miles of driving. The car is running fine. Any ideas of what to try next? How about injector cleaner?

That CIL just keeps staring at me, mocking me!

 

Injector cleaneror seafoam is not a bad idea since this should be done about every oil change. Are you positive that you replaced the sensor with the correct 1, and the other sensor dont need to be replaced?

 

I was told to always replace both 02 sensors at the same time...if for example the front sensor went bad and you replaced it, the rear sensor might be old as well and thus sending different flow signals. All in all it could make the car think the catalytic converter is bad due to one clean sensor and one dirty sensor sending mixed signals.

The sensors are expensive, but rarely do they need to be replaced so I'd advise doing both at the same time. Just my .02 cents!

LampyB.

 

Check the formed vacuum hoses,bothL and Y shaped ones.

 

Thanks a lot. Yes, it's probably good practice to do both 02 sensors at the same time. I read a lot of posts saying that the second one wasn't as importaint, so didn't bother. I will try that soon though if nothing else clears it up.

Pretty sure I bought the right one, I bought:
Audi 2001 Bosch (OEM) Oxygen Sensor Price: $136.95 Qty: 1 Part #: C5010-112690
from:
http://replacement.autopartswarehous...or&dp=true

I'll get some injector cleaner tonight.

which are the 'L and Y formed' hoses? Are they the small braided ones on the drivers side of the manifold?

 

+1 on checking the vacuum lines

 

Idont think you would get that code from the second O2 being bad.

I would look elsewhere first, definitely look for vacuum leaks.

Lean, not enough fuel, maybe try some injector cleaner, as mentioned.

 

In my post above I didn't mean to put forth the idea that his problem is due to replacing only one 02 sensor...I just wanted to mention that it's generally a good idea to do both if you're going to do one!

Sorry for the confusion...


LampyB.

 

The L shaped hose is between the G-12 reservoir and the intake manifold.
The Y shaped one is behind the valve cover near theL shaped hose.