A method of supplying fuel vapor to an exhaust aftertreatment device of a motor vehicle is provided. The method includes evaporating fuel on a voltage-biased, resistive heating element, the element set in an enclosure configured to release fuel vapor to the exhaust-aftertreatment device, and indicating that the element is degraded if a current flowing through the element is less than a threshold current. In some embodiments, the threshold current is based on the ambient temperature and/or the amount of fuel in the enclosure.