Thermal flowmeter designed to determine and/or monitor the mass flow (Φ M) and/or the flow velocity (v D) of a medium that can flow through a pipe with at least three sensor elements (4a, 4b, 4c) and an electronics unit, wherein each of the at least three sensor elements (4a, 4b, 4c) - are at least partially and/or temporarily in thermal contact with the medium and - comprise a heatable temperature sensor (5a, 5b, 5c), and wherein the electronics unit is designed to - heat each of the three sensor elements (4a, 4b, 4c) with a heating power (P1, P2, P3) - measure their temperatures (T1, T2, T3), - simultaneously heat at least two of the at least three sensor elements (4a, 4b, 4c), wherein at least one of the three sensor elements remains unheated and is used to measure the temperature of the medium, - continuously determine the mass flow (Φ M) and/or the flow velocity (v D) of the medium, and at the same time, - to evaluate - on the basis of a pair-based comparison of the temperatures (T1, T2, T3) and/or the heating powers (P1, P2, P3) of at least three sensor elements (4a, 4b, 4c) and/or of at least one variable derived from at least one of the temperatures (T1, T2, T3) and/or heating powers (P1, P2, P3) - a change in the thermal resistance of at least one of the at least three sensor elements (4a, 4b, 4c), - to evaluate - on the basis of a step response to a sudden change in the heating power supplied ΔP to at least one of the at least three sensor elements - a change in the internal thermal resistor of the at least one sensor element, and - to perform a correction of the measured value for the mass flow (Φ M) and/or the flow velocity (v D) and/or to generate and output a message on the condition of the at least one sensor element in the event that a change of the internal and/or external thermal resistor occurs at at least one of the at least three sensor elements (4a, 4b, 4c). wherein each of the at least three sensor elements (4a, 4b, 4c) - are at least partially and/or temporarily in thermal contact with the medium and - comprise a heatable temperature sensor (5a, 5b, 5c), and - are at least partially and/or temporarily in thermal contact with the medium and - comprise a heatable temperature sensor (5a, 5b, 5c), and wherein the electronics unit is designed to - heat each of the three sensor elements (4a, 4b, 4c) with a heating power (P1, P2, P3) - measure their temperatures (T1, T2, T3), - simultaneously heat at least two of the at least three sensor elements (4a, 4b, 4c), - heat each of the three sensor elements (4a, 4b, 4c) with a heating power (P1, P2, P3) - measure their temperatures (T1, T2, T3), - simultaneously heat at least two of the at least three sensor elements (4a, 4b, 4c), wherein at least one of the three sensor elements remains unheated and is used to measure the temperature of the medium, - continuously determine the mass flow (Φ M) and/or the flow velocity (v D) of the medium, and at the same time, - to evaluate - on the basis of a pair-based comparison of the temperatures (T1, T2, T3) and/or the heating powers (P1, P2, P3) of at least three sensor elements (4a, 4b, 4c) and/or of at least one variable derived from at least one of the temperatures (T1, T2, T3) and/or heating powers (P1, P2, P3) - a change in the thermal resistance of at least one of the at least three sensor elements (4a, 4b, 4c), - to evaluate - on the basis of a step response to a sudden change in the heating power supplied ΔP to at least one of the at least three sensor elements - a change in the internal thermal resistor of the at least one sensor element, and - to perform a correction of the measured value for the mass flow (Φ M) and/or the flow velocity (v D) and/or to generate and output a message on the condition of the at least one sensor element in the event that a change of the internal and/or external thermal resistor occurs at at least one of the at least three sensor elements (4a, 4b, 4c). - continuously determine the mass flow (Φ M) and/or the flow velocity (v D) of the medium, and at the same time, - to evaluate - on the basis of a pair-based comparison of the temperatures (T1, T2, T3) and/or the heating powers (P1, P2, P3) of at least three sensor elements (4a, 4b, 4c) and/or of at least one variable derived from at least one of the temperatures (T1, T2, T3) and/or heating powers (P1, P2, P3) - a change in the thermal resistance of at least one of the at least three sensor elements (4a, 4b, 4c), - to evaluate - on the basis of a step response to a sudden change in the heating power supplied ΔP to at least one of the at least three sensor elements - a change in the internal thermal resistor of the at least one sensor element, and - to perform a correction of the measured value for the mass flow (Φ M) and/or the flow velocity (v D) and/or to generate and output a message on the condition of the at least one sensor element in the event that a change of the internal and/or external thermal resistor occurs at at least one of the at least three sensor elements (4a, 4b, 4c).