Nissan Sentra Owners Manual: Engine coolant temperature gauge
The ignition switch must be placed in the ON position for the gauge to give a reading.
The gauge indicates the engine coolant temperature.
The engine coolant temperature is within the normal range when the reading is within the zone A shown in the illustration.
The engine coolant temperature varies with the outside air temperature and driving conditions.
If the gauge indicates a coolant temperature near the hot (H) end of the normal range, reduce vehicle speed to decrease the temperature. If the gauge is over the normal range, stop the vehicle as soon as safely possible. If the engine is overheated, continued operation of the vehicle may seriously damage the engine. See “If your vehicle overheats” in the “In case of emergency” section for immediate action required