The DME will set a "heat" code if it doesn't see o2 voltage respond properly during the heat cycle which can have nothing to do with the heater itself. The DISA valve is vacuum actuated so powering the valve that controls the vacuum when there is no vacuum won't show anything.

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