BMW temp gauges don't actually read the engine temperature directly.
It displays a readout from the engine computer, which knows exactly
how hot it is from it's sensors. Newer engines even have the computer
controlling the thermostat.
This is all so that the warm-up time is closely controlled and as short
as possible to reduce emissions.
AFAIK, there is no way to "bypass" it without screwing up the
emissions control system.
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