Patent ID: 8826648

Claim:
A method for selective catalytic reduction of nitrogen oxides in exhaust gas of an internal-combustion engine of a motor vehicle, wherein reducing agent is fed into the exhaust gas from a reducing agent tank internal to the vehicle, the method comprising the acts of: detecting, by way of a signal of a sensor in the reducing agent tank, whether the reducing agent is in a frozen state; applying heating current to a PTC heating element in the reducing agent tank if the reducing agent is in a frozen state; calculating a resistance of the PTC heating element from an onboard voltage of the motor vehicle and a heating current, and comparing the resistance with at least one of a first defined resistance threshold and with a first defined gradient threshold of an increase in resistance; and reducing the amount of reducing agent fed into the exhaust gas from the reducing agent tank if said at least one of the first resistance threshold and the first gradient threshold is exceeded.