Volkswagen Repair: Po171 code on my 2001 Jetta, obd ii reader, mass air flow sensor

Question
I bought an OBD II reader after my check engine light came on. accidentally erased it , it stayed off for a day, then came bck on with the same code. I have a 2001 Jetta gls 4 cyl. 2.0 L engine with 64,500 miles. It came on @ about 61,500, and has seemed to run fine since it came on. My brother who is a backyard mechanic says I need to replace the oxygen sensor. What do you think? If this is so, where would Find the part, do I need a nuber, and how do I replace it.  I am not unhandy with a wrench, just don't know how to do it without instructions. Thanks!

Answer
Hi MaryAnne;
Your brother sounds very helpful.  The P0170 code is for "system too lean" which could be caused by an oxygen sensor, but most Volkswagens from 2001 with a 2.0 liter AVH, or AEG, or even AZG engine codes(on the sticker on the timing belt cover), will develop this code for a mass air flow sensor that needs replacement.  It is right next to the air box where the air filter goes.  Attached to it in fact.  Replace the air filter at that time, and use a Volkswagen brand one.  The cheap ones don't seal around the edges of the air box, and the expensive ones coat the mass air flow sensor with oil, and it can't read correctly after a few thousand miles.  Do them both at once, clear the code, and drive.  If it hasn't returned in a week you should be good for several thousand more miles.  If it does come back on after a few thousand miles at that time, I would look at the catalyst.  They go bad too.  Oxygen sensors seem to last through a lot of driving, even during poor drivability conditions.  Good luck, MaryAnne, I hope this helps you get the problem fixed.