Eppy, I had to replace the one in my 2000 E320 and it was no where near $250. I would get a quote from an indie, that's what I did and was able to get one for around $125 if I remember correctly. I plugged the new one in before I installed it and all worked normally. If it didn't fix the problem I could have taken it back but luckily it fixed the problem. If you replace it yourself I can tell you from experience it helps to have another set of hands when you do it. Getting the sensor in place, getting the clips to hook, etc. etc. is really a pain if you are going to do it yourself.

Oh and the OBDII reader will not tell you SRS codes but it sounds like you've got it narrowed down to the problem due to your light going on and off when someone sits in the passenger seat.

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86 Nissan 720 P/U - 285K miles and counting!!
91 Volvo 240 - 218K miles and still going strong.
00 Mercedes E320 - 111K miles and lovin it!!!