A vehicle braking system comprising: a master cylinder having an input side coupled to a brake pedal and an output side configured to provide a master cylinder output via a first outlet and a second outlet, a first braking circuit having at least one wheel cylinder coupled to the first master cylinder outlet via a first normally-open isolation valve and a first normally-open inlet valve; a second braking circuit having at least one wheel cylinder coupled to the second master cylinder outlet via a second normally-open isolation valve and a second normally-open inlet valve; a brake pressure generator separate from the brake pedal; a simulator circuit including a pedal feel simulator coupled to the first master cylinder outlet through a switchable simulator valve, the simulator circuit being isolated from the first braking circuit when the first isolation valve is in a closed position, wherein the pedal feel simulator provides a reaction force to the brake pedal when the switchable simulator valve is in an open position; a pressure sensor positioned in the first braking circuit between the first isolation valve and an output of the brake pressure generator; characterised by a controller programmed to, at a designated diagnostic time when the brake pedal is depressed to generate a brake pedal input while the vehicle is locked out from being driven by one or both of a parking brake and a transmission parking gear, open the simulator valve without closing the first isolation valve to place the pedal feel simulator in fluid communication with the pressure sensor of the first braking circuit, and to observe with the pressure sensor an increase in brake fluid pressure resulting from the brake pedal input, wherein the controller is further programmed to check whether the relationship between the observed brake fluid pressure increase and the brake pedal input is within a predetermined acceptable range for continued operation of a brake-by-wire vehicle braking mode in which the master cylinder is coupled to the simulator circuit and decoupled from the first and second braking circuits, while brake fluid pressure to the wheel cylinders of the first and second braking circuits is provided solely by the brake pressure generator. a master cylinder having an input side coupled to a brake pedal and an output side configured to provide a master cylinder output via a first outlet and a second outlet, a first braking circuit having at least one wheel cylinder coupled to the first master cylinder outlet via a first normally-open isolation valve and a first normally-open inlet valve; a second braking circuit having at least one wheel cylinder coupled to the second master cylinder outlet via a second normally-open isolation valve and a second normally-open inlet valve; a brake pressure generator separate from the brake pedal; a simulator circuit including a pedal feel simulator coupled to the first master cylinder outlet through a switchable simulator valve, the simulator circuit being isolated from the first braking circuit when the first isolation valve is in a closed position, wherein the pedal feel simulator provides a reaction force to the brake pedal when the switchable simulator valve is in an open position; a pressure sensor positioned in the first braking circuit between the first isolation valve and an output of the brake pressure generator; characterised by a controller programmed to, at a designated diagnostic time when the brake pedal is depressed to generate a brake pedal input while the vehicle is locked out from being driven by one or both of a parking brake and a transmission parking gear, open the simulator valve without closing the first isolation valve to place the pedal feel simulator in fluid communication with the pressure sensor of the first braking circuit, and to observe with the pressure sensor an increase in brake fluid pressure resulting from the brake pedal input, wherein the controller is further programmed to check whether the relationship between the observed brake fluid pressure increase and the brake pedal input is within a predetermined acceptable range for continued operation of a brake-by-wire vehicle braking mode in which the master cylinder is coupled to the simulator circuit and decoupled from the first and second braking circuits, while brake fluid pressure to the wheel cylinders of the first and second braking circuits is provided solely by the brake pressure generator.