Coolant Temperature Sensor
WARNING: This page is about a different car, the 2010 Volvo V70, 2009 Volvo V70, and 2008 Volvo V70. However, it is still accessible from the selected car via links, so may be relevant.
The coolant temperature sensor enables the control module to determine coolant temperature and thereby when the heater should be switched on or off.
The coolant temperature sensor is integrated in the heater, located next to the overheating sensor under a cover on one side of the heater.
There are diagnostics for the electronics of the coolant temperature sensor.