Actually...it depends on which obc you have.

The sensor in the bottom of the expansion tank is a level sensor for the "high" obc. I had the same question when my tank just cracked as well. When I was taking the cracked tank out I noticed that my sensor was not hooked up at all. I have the basic obc on my car. After searching here it turns out that it is not used in the "low" obc cars. So, naturally, I didn't replace it!

Carter

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-Carter Siegfried
'02 Ti Silver X5 3.0 -- 5 spd 12/11/10
'99 Imola M Roadster -- hibernating for the winter
'89 Alpine 325ix -- sold 12/23/10
'89 Schwarz 750iL -- Sold 3/30/06
'86 Delphin 325es -- Sold 12/20/04