A method for checking an exhaust gas cleaning plant connected to an internal combustion engine in an exhaust gas tract that includes a catalytic converter having oxygen storage characteristics as well as a NOx sensor positioned downstream in the exhaust gas tract that emits a sensor signal depending on the NOx and the NH3 concentration in the exhaust gas in which case the internal-combustion engine is operated with a periodic air/fuel ratio oscillating around the value lambda=1 and that the average of the sensor signal is formed over one period or several periods and if the threshold value of the average is exceeded, damage to the monolith of the catalytic converter is detected.