A cover body of a vehicle air bag apparatus, being comprised of a covering portion which is formed integrally with an interior panel body and which has a door portion defined by a tear seam formed in a back of the interior panel body, and, a backing portion joined to a back of the covering portion. The method includes the steps of arranging a bypass duct bypassing the particle separator, and conducting a partial flow of exhaust gas along the duct for increasing the fly ash content in the exhaust gas after the separator. The sensor is coupled to a security system, which is activated on the triggering of the sensor.