IN THE WATER JACKET.
Coolant Temperature Sensor
Low coolant would be because the coolant is low or the sensor has failed, Check engine light would be because the computer has detected a problem and set a code.
Make sure you have engine coolant. If the engine isn't getting enough coolant, it will overheat, stall and possibly destroy the engine. One symptom is that no coolant is circulated into the heater. Also check the crank sensor, but check coolant first...then the crank sensor.
A coolant sensor could be a liquid level sensor and the other is a temp sensor OR The coolant is different temperatures at different places inside the engine and way way cooler when it comes out of the radiator so where the two sensors are mounted can cause difference's. OR they are two different pieces and calibration will not be identical and they will produce slightly different readings.
(based on a 96 civic LX) I believe the engine coolant temperature sensor is located on the side of the engine (opposite of the camshaft). If you need more details, I would recommend you to download the service manual on www.manualnguide.com
That would be the engine coolant temp sensor for the dash gauge only. The sensor near the thermostat housing is the engine temp sensor for the ECM.
One sensor would be for your engine temperature gauge and the other sensor for the OBD II engine management system
Bad coolant sensor or faulty wiring to the sensor.
50 50 coolant is what i use- would suggest no more han a 60 40 mix.
Coolant level would be a sensor going into the fill cannister on the left side of the engine bay. Temperature would be done with the sensor just below the EGR valve on the engine just above the transmission bellhousing. Tiny little wire going into a single plug sensor.
The engine coolant temp sensor can do that. If you have a check engine light on, have the codes read.