A method, comprising: setting a diagnostic dosing command for injection of reductant at an ammonia to oxides of nitrogen (NO x) ratio rate that is substantially less than a stoichiometric ANR rate; injecting reductant at the diagnostic command; determining a time for ammonia storage and urea deposit depletion based on a temperature of a selective catalytic reduction catalyst and a flow rate of an exhaust gas; depleting reductant deposits and reductant storage of the SCR catalyst with the exhaust gas from an internal combustion engine for the determined time while injecting reductant into the exhaust gas upstream of the SCR catalyst according to the diagnostic dosing command; while providing reductant to the SCR catalyst at the reductant dosing command and after depleting reductant storage with the exhaust gas: determining a measured engine-out NO x amount; determining an estimated system-out NO x amount from the measured engine-out NO x amount upstream of the SCR catalyst and the ANR rate; determining a measured system-out NO x amount from a system-out NO x sensor downstream of the SCR catalyst; determining a gain error for the system-out NO x sensor based on a ratio of the measured system-out NO x amount to the estimated system-out NO x amount; determining a fault condition of the system-out NO x sensor in response to the determined gain error being below a threshold value; and providing the fault condition to a diagnostic output device. setting a diagnostic dosing command for injection of reductant at an ammonia to oxides of nitrogen (NO x) ratio rate that is substantially less than a stoichiometric ANR rate; injecting reductant at the diagnostic command; determining a time for ammonia storage and urea deposit depletion based on a temperature of a selective catalytic reduction catalyst and a flow rate of an exhaust gas; depleting reductant deposits and reductant storage of the SCR catalyst with the exhaust gas from an internal combustion engine for the determined time while injecting reductant into the exhaust gas upstream of the SCR catalyst according to the diagnostic dosing command; while providing reductant to the SCR catalyst at the reductant dosing command and after depleting reductant storage with the exhaust gas: determining a measured engine-out NO x amount; determining an estimated system-out NO x amount from the measured engine-out NO x amount upstream of the SCR catalyst and the ANR rate; determining a measured system-out NO x amount from a system-out NO x sensor downstream of the SCR catalyst; determining a gain error for the system-out NO x sensor based on a ratio of the measured system-out NO x amount to the estimated system-out NO x amount; determining a fault condition of the system-out NO x sensor in response to the determined gain error being below a threshold value; and providing the fault condition to a diagnostic output device. determining a measured engine-out NO x amount; determining an estimated system-out NO x amount from the measured engine-out NO x amount upstream of the SCR catalyst and the ANR rate; determining a measured system-out NO x amount from a system-out NO x sensor downstream of the SCR catalyst; determining a gain error for the system-out NO x sensor based on a ratio of the measured system-out NO x amount to the estimated system-out NO x amount; determining a fault condition of the system-out NO x sensor in response to the determined gain error being below a threshold value; and providing the fault condition to a diagnostic output device.