A vehicle mounted combined HVAC and breath analyzing system acting on a compartment of the vehicle, the combined HVAC and breath analyzing system comprising: - a breath analyzer arranged to analyze air from the compartment and to determine a concentration of a target substance in an air sample; - an HVAC system comprising HVAC system parts to control the air flow in the HVAC system and in the compartment, the HVAC system parts comprising at least one exterior air inlet, at least one interior air inlet and at least one interior air outlet, at least one fan and at least one valve, the HVAC system arranged to have plurality of modes of operation representing different ventilation, heating and cooling operations achievable by different settings or sequences of settings of the HVAC system parts, and the HVAC system having at least one open position drawing air from the exterior air inlet and at least one closed position drawing air from the interior air inlet, and in both the open and closed position deliver air to a compartment of the vehicle; comprising - a central control unit arranged to control both the breath analyzer and the HVAC system; wherein the central control unit is arranged to override a current mode of the HVAC system to place the HVAC system in a dedicated breath analysis mode, the breath analysis mode comprising controlling the settings of the HVAC system parts to be in positions that improves the possibilities for the breath analyzer to perform the target substance concentration determination, wherein the breath analysis mode comprises at least one time period in which the HVAC system is arranged to be in a closed position, the closed position not allowing any exchange of the air in the compartment with the exterior air and thereby causing a build-up of target substance concentration in the compartment and at least during that time period the central control unit arranges the breath analyzer to analyze air from the compartment, characterized in that the HVAC system is arranged to rapidly shift to an open position and wherein at least during the transition from the closed to the open position the central control unit arranges the breath analyzer to analyze air from the compartment. - a breath analyzer arranged to analyze air from the compartment and to determine a concentration of a target substance in an air sample; - an HVAC system comprising HVAC system parts to control the air flow in the HVAC system and in the compartment, the HVAC system parts comprising at least one exterior air inlet, at least one interior air inlet and at least one interior air outlet, at least one fan and at least one valve, the HVAC system arranged to have plurality of modes of operation representing different ventilation, heating and cooling operations achievable by different settings or sequences of settings of the HVAC system parts, and the HVAC system having at least one open position drawing air from the exterior air inlet and at least one closed position drawing air from the interior air inlet, and in both the open and closed position deliver air to a compartment of the vehicle; comprising - a central control unit arranged to control both the breath analyzer and the HVAC system;