Method of operating a flow sensor device, said flow sensor device comprising: at least one first sensor arrangement configured and arranged for measuring a flow of a fluid (g) and at least one further first fluid property (p 1); at least one second sensor arrangement in contact with said fluid (g) and configured and arranged for measuring at least one further second fluid property (p 2); wherein at least one of said at least one further first fluid property (p 1) corresponds to one of said at least one further second fluid property (p 2) such as to enable a comparison by means of said flow sensor device; wherein said further first fluid property (p 1) and/or said further second fluid property (p 2) are at least one parameter selected from the group consisting of: the thermal conductivity ( λ) of said fluid (g), the volumetric specific heat ( c p ·ρ) of said fluid (g), and the thermal diffusivity ( α) of said fluid (g); and said method comprising the steps of: a) to operate said flow sensor device for determining said flow of the fluid and said at least one further first fluid property (p 1) by means of said first sensor arrangement; b) to operate said flow sensor device for determining said at least one further second fluid property (p 2) by means of said second sensor arrangement; c) to compare at least one of said at least one further first fluid property (p 1) and at least one of said at least one further second fluid property (p 2) with one another and to produce a comparison result; d) to monitor said comparison result and to produce a fault signal if said comparison result deviates from a predetermined value or is outside a predetermined value range (V R). at least one first sensor arrangement configured and arranged for measuring a flow of a fluid (g) and at least one further first fluid property (p 1); at least one second sensor arrangement in contact with said fluid (g) and configured and arranged for measuring at least one further second fluid property (p 2); wherein at least one of said at least one further first fluid property (p 1) corresponds to one of said at least one further second fluid property (p 2) such as to enable a comparison by means of said flow sensor device; wherein said further first fluid property (p 1) and/or said further second fluid property (p 2) are at least one parameter selected from the group consisting of: the thermal conductivity ( λ) of said fluid (g), the volumetric specific heat ( c p ·ρ) of said fluid (g), and the thermal diffusivity ( α) of said fluid (g); and the thermal conductivity ( λ) of said fluid (g), the volumetric specific heat ( c p ·ρ) of said fluid (g), and the thermal diffusivity ( α) of said fluid (g); and said method comprising the steps of: a) to operate said flow sensor device for determining said flow of the fluid and said at least one further first fluid property (p 1) by means of said first sensor arrangement; b) to operate said flow sensor device for determining said at least one further second fluid property (p 2) by means of said second sensor arrangement; c) to compare at least one of said at least one further first fluid property (p 1) and at least one of said at least one further second fluid property (p 2) with one another and to produce a comparison result; d) to monitor said comparison result and to produce a fault signal if said comparison result deviates from a predetermined value or is outside a predetermined value range (V R). a) to operate said flow sensor device for determining said flow of the fluid and said at least one further first fluid property (p 1) by means of said first sensor arrangement; b) to operate said flow sensor device for determining said at least one further second fluid property (p 2) by means of said second sensor arrangement; c) to compare at least one of said at least one further first fluid property (p 1) and at least one of said at least one further second fluid property (p 2) with one another and to produce a comparison result; d) to monitor said comparison result and to produce a fault signal if said comparison result deviates from a predetermined value or is outside a predetermined value range (V R).