Mass Air Sensor - help please !!!!!!

Just found your site. It's great. I use one for my 325i and it is the best friend that a diy mechanic ever had. My new problem is my girlfriend's 99 320.
She has followed the dealer service schedule to the letter. All work ( even new tires ) performed by local dealer. At the 100k mark she had a service engine light come on. Dealer says "no problem, we can't find a code"
Next service @ 106k , light still on. Dealer says "no problem, no code, we changed the air filter"
Next service @ 112k , light still on. Dealer says "you have a problem, You need either a mass air sensor or an O2 sensor, I can
not tell which" Price estimate equals $ 1200 cdn.
My questions are:
Common Problem ?
Anyone know of any service bulletins or hidden warranties ?
Can I replace it myself ?
Where would you suggest I shop for the part ?

Thanks in advance for your help on this.

[Gilly]

Alvin:
There are specific codes which would lead one to want to replace either the MAF sensor or an O2 sensors. With the fuel injection you have, ME2, from my experience the O2 sensor circuit will usually fault a specific sensor, for various reasons.
On the MAF sensor, the fault USUALLY will be listed as "self-adaptation at limits". Self adaptation in other vehicle manufacturer nomenclature is also called "block learn", what it is is setting the parameters that the fuel injection mixture will work within. The area it will work in will vary to the rich or lean side, it will shift or vary depending on operating conditions. If it shifts too far rich or lean, eventually it will not be able to shift anymore in that direction and will have to stop.
Very often the MAF is the culprit, it will be sending incorrect info to the ME control module. The dealer would have this code to go on, plus they can look and see what the self-adaptation number is.
1.00 is ideal, 1.32 is the limit in the lean side, -0.68 is the limit on the rich side (+- .32). Usually they fail to the lean side, I've seen them fail to the rich side. When I see this code I normally assume it's the MAF sensor, but you have to check for a vacuum leak as this can cause this failure, or any intake leak after the MAF sensor can cause it, as unmetered air will be getting into the induction system. The sensor is easy to replace, BUT after replacing it the self-adaptation should be cancelled ie brought back to 1.00 and roadtested several miles and the self-adaptation checked to make sure it is staying fairly well in the middle of the range. The code or codes should be erased as well.
My only problem with your scenario of course is the fact that the dealer seems indecisive on what the problem is. If you could ask the dealer what the code is maybe I could be of further help. Or if they say "self adaptation is at the limits", or to that effect, I would say based on my experience, I'm 95% sure it's the MAF sensor. The O2 sensor problems again are usually quite easy and shouldn't be causing any problem for the dealer to diagnose.
I also find it strange that they claim the first two visits had no codes at all. The check engine light doesn't come on for "no" reason, or if it does I'd say the ME control unit is bad. There are other control units which ultimately can make the check engine light come on, but there would be other lights on as well, such as the ETS or BAS lights, but there would still be a reference code in the ME control unit, basically saying "Hey, I turned the light on, but I didn't cause it, look at the "fill in the blank" control unit!".
Gilly