I just saw the check engine light come on on my 2004 VW Passat B5.5 and did a bit of research. Some people said that it would be a faulty oxygen sensor. I was about to order one, but then I read this (from here):
Next, looking at P0172, which is telling us bank 1 is reading a too rich condition. It is reporting that the engine is running too rich, which is what we'd expect if smelling un-burned fuel out the exhaust. This, too, is not the root cause and should be cleared up when you solve the primary issue. You may find, however, since there was an extreme amount of fuel dumped into the system, the O2 (I'm assuming this is your new sensor as well) may have been damaged in the process. This may come back to life after the excess fuel quits dumping.
That was in relation to another issue, however it confused me enough to doubt whether buying a new oxygen sensor would in fact fix the problem!
How do I fix the P0172 trouble code?
Also, if it is an oxygen sensor - are they all the same? Can I order the wrong one for my car or can I just get the cheapest?
UPDATE I got some more info off an app on my smartphone connected to my OBDII port. I see O2 sensor is N/A! That means that I can't see how lean the engine is running - that should mean it's the sensor that's broken now right?
Here's a screenshot:
Wow, sorry its so big!