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Q:

What could cause a temperature gauge to malfunction?, even if the car is running on 1991 BMW 325i

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Don't know how much more to add other than what was told, its' an electrical problem. Work has been done, but the car hasn't been driven as of yet.
(1) Answer
Engine temp gauge or outside air temp gauge? If you're talking about the outside air temp, the sensor for it is located under the front of the vehicle and they are easily damaged by parking stops, curbs etc.. due to their location.