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To test if your sensor is bad, get yourself a $10 infrared thermometer. When you've driven that 30 minutes, pull over, pop the hood and shoot the thermometer at the side of the cylinder head by the glow plugs and such. See if it's really reading 40C. If not, and you're seeing more like 80-90C, everything else is fine and the reading is bad - perhaps due to the sensor, gauge, cluster grounding, etc.
If you find the reading is correct and your engine is really that cool then your thermostat is stuck wide open and needs to be replaced.
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