Abstract: A vapor sensor is disclosed, which includes a substrate containing a sensing element, wherein the substrate is located proximate to a sleeve portion which covers and protects the sensing element. The vapor sensor also includes a vapor filter or permeable membrane affixed to an end of the sleeve portion, wherein the end of the sleeve portion is located opposite the sensing element. A housing and a cover are also provided, such that the housing retains the substrate and the cover snaps onto the housing. The housing itself includes walls from which a hole is formed through which vapors may pass. The vapor sensor can be mounted to a gas water heater in such a manner as to reduce air flow to the sensing element and prevent tampering of the vapor sensor unit itself.