Located on the water rail, which runs from the rear of the exhaust side of the cylinder head to the radiator top hose (brown connector).
The CTS is a negative temperature coefficient thermistor (electrical resistance drops with temperature increase). The ECU measures the voltage drop across the CTS to establish the coolant temperature.
By measuring the coolant temperature returning to the radiator, the ECU can adjust the idle speed and the air fuel ratio to suite ie. at cold start. Should this sensor fail the ECU will assume a temperature of 60°.