Are you measuring voltage across the heater element? or referencing ground or supply? According to the wiring diagram the DME grounds the heating element. If you disconnect the sensor long enough to generate a sensor code you could verify if you are working on the right one.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)