BMW: O2 Sensor/Check Engine Light, blue zone, air mass

Question
In the past month and a half I have asked you about a recurring problem that I had with my 1995 525.  To refresh your memory, my engine light stayed on most of the time during Nov/Dec. The dealership replaced the O2 sensor twice, but the engine light wouldn't go off. They thought the problem might be the Air Mass Meter, or the ECU.  Anyway, after educating myself by reading through several of your answers to other people about similar problems, I noticed that my thermostat was stuck in the blue zone, too.  Since I can't work on the car myself, I went back to the dealership and they replaced the thermostat.  Unfortunately, the engine light continued to stay on, but I was scheduled to come back to replace the O2 sensor "after being fitted" at this appointment.  Just a few days ago, the O2 sensor was replaced a third time under warranty by the same dealership.  This satisfied the CEU and the check engine light has stayed off.  Frankly, I don't know what happened, but I was told that the O2 sensor was "defective."  Although it took several attempts to correct the problem, I was pleased with the result and the goodwill of the dealership. Because of your support, and knowledge about BMW's, and your interest in helping people you don't even know, I saved several hundred dollars, learned alot, and still have my beloved 1995 525 in excellent running condition.  Thank you so much.

Answer
Teresa,
 Thank you for your kind words, It makes me happy to know that I have helped someone save money, especially this time of year.  Most of all I'm glad that I could be of assistance and helped you learn about the car that you own.  There's nothing better than having a problem, and knowing why/how it happend, and how to fix it.

 It is unfortunate and dissapointing that a dealership, of all places, cannot get your car fixed correctly, the first time.  I have noticed that most of the parts salesmen there have a VERY limited knowledge of the cars they are selling parts for.  I basically have to give them part numbers to make sure they order the right part for me.

 Once again, thank you for what you said, and I hope you had a Merry Christmas (or other Holiday if you celebrate one).  If you have future questions, please do not hesitate to ask them.

 Take care,
 Josh