METHOD AND DEVICE FOR DETECTING THE OPERATION OF AN ENGINE-INDEPENDENT AUXILIARY HEATING AND FOR DIAGNOSING A COOLANT TEMPERATURE SENSOR OF A MOTOR VEHICLE

Method and Device for Detecting Operation of an Engine-Independent Auxiliary Heater and for Diagnosing a Coolant Temperature Sensor of a Motor Vehicle The invention relates to a method for detecting operation of an engine-independent auxiliary heater in a cooling system (2) with multiple coolant circuits (K1, K2, K3) in a vehicle system with an auxiliary heater (6), comprising the following steps: - Upon or after activating the vehicle system for a cold start, opening (S2) a blocking valve (9) between a first coolant circuit (K1) with the auxiliary heater (6) and a second coolant circuit (K2) with the coolant temperature sensor (12) and activating a coolant pump (8), so that coolant from the first coolant circuit (K1) flows around the coolant temperature sensor (12); - Detecting (S3) a time course of a temperature measured value of the coolant temperature sensor (12) for a predetermined duration from the time point of activating the vehicle system; - Determining (S4) a course of a gradient of the time course of the temperature measured value; - Evaluating (S5, S6) the course of the gradient of the time course of the temperature measured value using a threshold value comparison in order to determine whether the auxiliary heater (6) was activated before the cold start.