Coolant Temperature Sensor CTS

Article describes how a engine coolant temperature sensor works.

An coolant temperature sensor is located at the water outlet of the an internal combustion engine. Comprised of a housing filled with a composite which varies the resistance through the internal circuit (2 wire.) The car's computer or PCM uses this reading to make adjustments to the engine and transmission operating systems. Also described as an ECTS (electronic coolant temperature sensor) these sensors are easily tested using a voltmeter that can read the ohms of resistance at a particular engine temperature. This sensor also provides data to adjust the in-dash gauge to inform the driver on engine operating temperatures. In older models this operation was perform by a coolant sensor which was a one wire sensor that performed the gauge readings solely.

Coolant Temperature Sensor ECTS

When the engine is started, the computer uses sensor data feedback to set initial adjustments for timing and fuel delivery. When this sensor fails it will set a trouble code which is indicated by a service engine soon or check engine warning light. Visit - Code Scan

This sensor can cause problems as far as run-ability problems when the sensor becomes internally compromised. This condition will cause the computer to act on false data which causes a rich or lean condition for the engine. These problems can be particularly difficult to find unless a manual test of the sensor is carried out. Visit - Car repair manual

A coolant leak can cause a false reading as well due to the lack of liquid in the cooling system. Visit - Coolant leak

If the engine's cooling thermostat fails by sticking in the open position, it can sometime cause the computer to detect a coolant sensor failure after a sufficient amount of time has passed.  When replacing a coolant sensor use plumbers tape to seal the sensor threads to avoid coolant leaks. Also, never install the sensor with force, theses sensor are delicate and can be damaged when over tightening occurs.


Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2015-01-07)