Abstract: The vapor-sensing protective system of the present invention has a sensing device for fuel vapor which is mounted in an area where a fuel-powered engine is located. That sensing device develops a voltage within a predetermined range whenever it is operative and is properly connected in that system and senses for but does not detect fuel vapor; and that system responds to that voltage to permit starting of that engine. If that sensing device is not operative or is not properly connected in that system, it will be unable to develop a voltage within that predetermined range; and hence that system will prevent starting of that engine. If that sensing device is operative and is properly connected in that system and senses for and detects fuel vapor, it will develop a voltage above that predetermined range; and hence that system will prevent starting of that engine.