The thermostat has a heating element in it. Check the impedance, I think it should be about about 12 ohms. If you have GT1 there is procedure to follow to identify the problem but it looks to me that the circuit is open to the heating element.

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4.8is 2004 imola red
4.8is 2005 Black on Black - DINAN/2 sold
4.4 2004 X5
2003 330xi
2008 328xi - sold
4.4 2000 X5 - sold