BMW: Check Engine Light, mass air sensor, air flow meter

Question
A week ago you answered my questions about the O2 sensor on my 1995 525i.  I understand that your expertise is related to the e30, but this next question is probably related to most cars in general.  The BMW mechanic replaced the O2 sensor for no charge, but the engine light still comes on.  I am told that although the O2 sensor code is still showing that it needs to be replaced, the service representative told me that it is something else.  I didn't get a written quote so I don't know what this part is, but he pointed it out to me.  My question is what is the big rectangular plastic box that is on the drivers (US specs) side of the car and is connected by a 3-4 inch diameter hose to the engine?  I think the air filter is inside of it.  I am told that it will take 15 minutes to replace it and cost about 300 pounds ($600).

Answer
Well, if you replaced all the O2 sensor(s) on the car, and you are still getting a check engine light, then have them tell you EXACTLY what the code says.  
 The part you said, sounds like the air filter.  This a box, that is usually behind the headlight, and has a large diameter pipe that runs to the throttle body (on engine).  This is what your engine breathes through, and there is a filter in that big box.  That sounds exactly like what you described.  
 Behind this big box is a smaller boxy looking aparatus, that is the Air flow meter, or Mass air sensor.  This tells the computer in the car how much air is entering the engine, and it's temperature. The computer responds accordingly by adding fuel or decreasing fuel to match the ammount of air coming in.  This could cost $300.00 and would only take a few minutes to install.  I am always wary of repair shops, because if you have a flat tire, they will try to sell you 4 new ones, instead of fixing one.  In this case they "could" have gone over-board and just suggested this Mass Air sensor was the culprit, so I would ask to personally see what their tool is saying that the problem is.  It should show a 4-digit code, and this is what the mechanics are looking at to tell them what is wrong.  So if you get this code, or an exact read-out of what the problem is, you can tell me the code and I will personally check for you.
 I just dont want to see you spend money that isn't necessary.
 Take care,
 Josh