The invention relates to a method (200) for determining (290) the amount of NH3 contained in an
exhaust gas (10) downstream of an
internal combustion engine (1) using an NH3 and
NOx sensitive NX sensor (17, 18, 19), comprising receiving (210) a sensor
signal from the NX sensor and determining the amount of NH3 based on the sensor
signal and depending (220, 240, 260) on the fulfillment of at least one condition (331, 332, 333), wherein the at least one condition (331, 332, 333) is the presence of an
exhaust gas temperature exceeding a minimum
exhaust gas temperature, in particular for the first time, as a first condition, and / or the presence of an exhaust gas temperature exceeding a maximum exhaust gas temperature as a second condition, and / or the presence of heat input into an exhaust gas
system (120) upstream of the NX-Sensors (17, 18, 19), which exceeds a minimum heat input, as the first condition,and / or the presence of a heat input into an exhaust
system (120) upstream of the NX sensor (17, 18, 19) that exceeds a maximum heat input, as the second condition. In embodiments of the invention, further conditions can also be checked, wherein, in particular, the presence of a sensor
signal level that exceeds a minimum level can be used as a third condition, and / or the presence of a sensor signal gradient that exceeds a positive threshold can be used as a fourth condition. Furthermore, a computing unit (20) and a
computer program for carrying out such a method (200) are proposed.