A procedure for a reasonability check of the signal of a temperature sensor that is arranged in a reducing tank, in particular a HWL-tank, is thereby characterized, in that the tank level is detected by a tank level sensor that is arranged almost at the same filling level position as the temperature sensor, in that the tank level sensor signal is compared to the temperature sensor signal and then, if default tank level sensor signals correlate within default limits with default temperature sensor signal values, a functioning temperature sensor and/or a functioning tank level sensor are indicated.