A method for fault identification of gas sensors exposed to a gas mixture, comprising the steps of: receiving a first output signal from a first gas sensor having an output that varies according to both the concentration of a first gas species in the gas mixture and the concentration of a second gas species in the gas mixture; receiving a second output signal from a second gas sensor having an output that varies according to both the concentration of the first gas species in the gas mixture and the concentration of the second gas species in the gas mixture; processing the first output signal and the second output signal in a diagnostic controller that implements a model of the first gas sensor and a model of the second gas sensor so as to identify a fault in the first gas sensor or the second gas sensor and, indicating any identified faults and, wherein the gas mixture comprises an exhaust gas from an internal combustion engine having an exhaust system comprising means for controllably introducing ammonia into the gas mixture, and wherein the first gas species is NH 3 and the second gas species is NO x characterized in that, the processing of the first output signal and the second output signal includes the steps of: using the first output signal and the second output signal as inputs to the model of the first gas sensor to calculate the concentration of the first gas species in the gas mixture; using the calculated concentration of the first gas species as an input to the model of the second gas sensor to calculate a predicted signal level from the second gas sensor; indicating a first fault condition if the second output signal is not within a first predetermined range; indicating a second fault condition if the predicted signal level is not within a second predetermined range; indicating a third fault condition if the difference between the second output signal and the predicted signal level is not within a third predetermined range. receiving a first output signal from a first gas sensor having an output that varies according to both the concentration of a first gas species in the gas mixture and the concentration of a second gas species in the gas mixture; receiving a second output signal from a second gas sensor having an output that varies according to both the concentration of the first gas species in the gas mixture and the concentration of the second gas species in the gas mixture; processing the first output signal and the second output signal in a diagnostic controller that implements a model of the first gas sensor and a model of the second gas sensor so as to identify a fault in the first gas sensor or the second gas sensor and, indicating any identified faults and, wherein the gas mixture comprises an exhaust gas from an internal combustion engine having an exhaust system comprising means for controllably introducing ammonia into the gas mixture, and wherein the first gas species is NH 3 and the second gas species is NO x characterized in that, the processing of the first output signal and the second output signal includes the steps of: using the first output signal and the second output signal as inputs to the model of the first gas sensor to calculate the concentration of the first gas species in the gas mixture; using the calculated concentration of the first gas species as an input to the model of the second gas sensor to calculate a predicted signal level from the second gas sensor; indicating a first fault condition if the second output signal is not within a first predetermined range; indicating a second fault condition if the predicted signal level is not within a second predetermined range; indicating a third fault condition if the difference between the second output signal and the predicted signal level is not within a third predetermined range. using the first output signal and the second output signal as inputs to the model of the first gas sensor to calculate the concentration of the first gas species in the gas mixture; using the calculated concentration of the first gas species as an input to the model of the second gas sensor to calculate a predicted signal level from the second gas sensor; indicating a first fault condition if the second output signal is not within a first predetermined range; indicating a second fault condition if the predicted signal level is not within a second predetermined range; indicating a third fault condition if the difference between the second output signal and the predicted signal level is not within a third predetermined range.