Method for monitoring a liquid level in a liquid tank of a vehicle, the liquid tank having a first ultrasonic sensor and a second ultrasonic sensor which are mounted at the bottom of the liquid tank, the first ultrasonic sensor being configured to detect the liquid level in the liquid tank by transmitting ultrasound towards the top of the liquid tank and receiving an echo signal, and the second ultrasonic sensor being configured to judge whether liquid is present below an ultrasound transmitting part of the first ultrasonic sensor by transmitting ultrasound along the bottom and receiving an echo signal, the method comprising the following steps: causing the first ultrasonic sensor to transmit ultrasound and receive a first echo signal, in order to detect the liquid level in the liquid tank; when the first ultrasonic sensor has not received the first echo signal for a period of time, causing the second ultrasonic sensor to transmit ultrasound and receive a second echo signal; judging whether the liquid tank is an empty tank on the basis of the second echo signal; determining that the liquid tank is an empty tank, when it is determined on the basis of the second echo signal that no liquid is present below the ultrasound transmitting part of the first ultrasonic sensor; wherein the liquid tank further comprises a heater for heating liquid in the liquid tank, the heater being arranged at a position higher than the ultrasound transmitting part of the first ultrasonic sensor; characterized in that the method further comprises the following step: when it is determined on the basis of the second echo signal that liquid is present below the ultrasound transmitting part of the first ultrasonic sensor, activating or maintaining heating of the liquid in the liquid tank by the heater, and monitoring a heating state of the heater; when it is determined that the heating state of the heater is stable heating, it is determined that the liquid tank is over-filled such that there is no interface between exhaust gas treatment fluid and air, and no ultrasound is reflected back to the ultrasonic sensor, so the first ultrasonic sensor is unable to detect the liquid level; and when it is determined that the heating state of the heater is unstable heating, it is determined that the liquid level in the liquid tank has fallen below the ultrasound transmitting part of the first ultrasonic sensor but has not been emptied. causing the first ultrasonic sensor to transmit ultrasound and receive a first echo signal, in order to detect the liquid level in the liquid tank; when the first ultrasonic sensor has not received the first echo signal for a period of time, causing the second ultrasonic sensor to transmit ultrasound and receive a second echo signal; judging whether the liquid tank is an empty tank on the basis of the second echo signal; determining that the liquid tank is an empty tank, when it is determined on the basis of the second echo signal that no liquid is present below the ultrasound transmitting part of the first ultrasonic sensor; wherein the liquid tank further comprises a heater for heating liquid in the liquid tank, the heater being arranged at a position higher than the ultrasound transmitting part of the first ultrasonic sensor; characterized in that the method further comprises the following step: when it is determined on the basis of the second echo signal that liquid is present below the ultrasound transmitting part of the first ultrasonic sensor, activating or maintaining heating of the liquid in the liquid tank by the heater, and monitoring a heating state of the heater; when it is determined that the heating state of the heater is stable heating, it is determined that the liquid tank is over-filled such that there is no interface between exhaust gas treatment fluid and air, and no ultrasound is reflected back to the ultrasonic sensor, so the first ultrasonic sensor is unable to detect the liquid level; and when it is determined that the heating state of the heater is unstable heating, it is determined that the liquid level in the liquid tank has fallen below the ultrasound transmitting part of the first ultrasonic sensor but has not been emptied. when it is determined on the basis of the second echo signal that liquid is present below the ultrasound transmitting part of the first ultrasonic sensor, activating or maintaining heating of the liquid in the liquid tank by the heater, and monitoring a heating state of the heater; when it is determined that the heating state of the heater is stable heating, it is determined that the liquid tank is over-filled such that there is no interface between exhaust gas treatment fluid and air, and no ultrasound is reflected back to the ultrasonic sensor, so the first ultrasonic sensor is unable to detect the liquid level; and when it is determined that the heating state of the heater is unstable heating, it is determined that the liquid level in the liquid tank has fallen below the ultrasound transmitting part of the first ultrasonic sensor but has not been emptied.