Interpreting OBDII Codes
 

Does anyone know where to go to get more information on what individual OBDII codes mean? The codes are often quite brief, like P0171 "System too lean (bank 1)". This happens to be the current code on a family member's 2001 Passat 1.8T (4 cyl. - only one bank) - which is running fine - but the code and check engine light keep coming back. It would seem like maybe a bad O2 sensor but there must be a dozen codes for O2 sensor - so why wouldn't a bad O2 sensor give an O2 sensor code?

It seems like somewhere there should be an analysis of the probable causes are for each of the codes.

Look for vc leaks

Arthur may be right, its a good place to start. If you can't find a vac leak, I still think its the sensor.
I wouldn't expect an O2 sensor code unless the continuity of the sensor is somehow compromised... i.e. broken wire or internal open of the circuit.
If you clear the code, my guess is that the light comes on and the code resets after about 45 seconds of running after a cold start. If so, what's happening is that the car is taking too long to go into "closed loop" operation after starting i.e. the O2 sensor is sluggish from age/contamination. Don't bother trying to clean it, its a waste of time in my opinion.
The ECM looks at the number of O2 cycles (crossings) in the first 45 seconds or so of running. If the sensor comes up too slowly, it will set the "too lean" code. Because the sensor eventually comes up, the car seems to run fine, but the code stays.
Find someone with a scanner that can watch the O2 sensor real time when initially starting and I bet you'll see how sluggish it is. If so, replace the sensor upstream of the cat.

02 sen codes on 0BD11 systems check for open, shorted , heater amp draw , and cross count , so a trim code without a 02 code is highly unlikely an indicator of a bad O2 sensor.
A lone trim code usually means the ECU is getting trim inputs , but it can not correct for them b/c they are out of the limitations of correction.
This can mean too much fuel [ like a bad fuel pressure regulator] or too much air [ like a vac leak or unmetered intake air to the engine, like an air leak after the MAF, between the intake and MAF]
.and finally, just a bad MAF not responding to the intake air mass .
Baically, the problem is a/f mix is beyond the capacity/limits of the system to correct...so a code pops telling you it is trying to correct an a/f mix problem, but the problem is beyond it's engine management parameters/limits to correct.....ie-getting too much air or fuel , depending on which trim code shows up...

There's no "Arthur might be right" to it. He IS right. There is a vacuum leak and probably one serious enough to hear.

Good luck,

Squirt some carb cleaner on the intake manifold gasket area. NOT the exhaust, fire is possbile.

Listen for improvements.

Check out VW forums. Talk to the service manager at a VW dealer.

I failed to mention that it is a 45k mile car and supposedly a 100k O2 sensor. I also failed to mention that the problem started a day or two after I had changed the spark plugs, air filter, etc. So I probably knocked a vacuum hose loose. The dealer had a change of ownership and the new owner has become ridiculously aggressive (crooked) on service. My sister is afraid to even go into the place. It may take a while but I'll let you know how it goes. Thanks for all the help.

<<I also failed to mention that the problem started a day or two after I had changed the spark plugs, air filter, etc. So I probably knocked a vacuum hose loose. >>

Well, in that case, not only look at vac lines , but also check the air tube that is between the engine and the MAF sensor I am not familiar with the VW set up , but if any clamps or other sensors [ like Air inake temp, etc.] plug into this tube, they have to be tight or back in place or else unmetered air [ meaning air that can get to engine without going through the MAF sensor to be measured ] will bring up your lean code condition. Go recheck the parts you know you took off when changing the plugs and such...
If you find nothing , then as W Bain says , VW forum will know more than us Benzheads on specifics ..give them a model/year and they may point you right to the exact cause , whereas here we are speaking generic 0BD2 possibles,,