The invention relates to a method for testing the operation of an electric heating element which is used for heating activated carbon of an activated carbon filter and/or air which is guided through the activated carbon for the regeneration thereof. The heating element has a PTC-characteristic (positive temperature coefficient characteristic). The strength of a current flow through the heating element in a heating phase selected for the operational testing is measured at a point in time or over a time period and is used for the operational testing, in that at least one measured current flow value is compared with a corresponding current flow value to be anticipated during defect-free operation of the heating element, and the heating element is considered to be defective in the event of a deviation which exceeds a predetermined measurement.A further method in accordance with the invention corresponds to the first-named method, but with the difference that an electric resistor which is not the heating element has a PTC-characteristic. The PTC-resistor is electrically connected to the heating element such that the current flowing through the heating element also flows through the PTC-resistor. The PTC-resistor is disposed in spatial terms adjacent to the heating element in such a manner that the heating element also heats the PTC-resistor.A further method in accordance with the invention corresponds to the second-named method, but with the difference that in any event the heating element serves to heat an air flow provided for the regeneration of the activated carbon and the PTC-resistor is disposed in a spatial relationship with respect to the heating element such that the PTC-resistor is heated by virtue of the fact that the regeneration air flow heats the PTC-resistor.