A system and a method for determining a cause for impaired performance of a catalytic configuration

The invention relates to a method for determining a cause for impaired performance of acatalytic configuration for emission control of exhaust gas of a combustion engine (231), saidcatalytic configuration being arranged for NO-conversion, comprising the steps of:- continuously or intermittently determining (s410) a course of a ΝΟ-conversion ratio;- continuously or intermittently determining (s420) a prevailing temperature of said catalytic configuration;- increasing (s430) the temperature of said catalytic configuration from a prevailing temperature below a to a predetermined temperature value (Te) to a temperature (TSred) above said predetermined temperature value, said predetermined temperature value (Te) representing a temperature above which sulphur, poisoning said catalytic configuration, is removed from said catalytic configuration so as to improve the performance of said catalytic configuration in case of sulphur, poisoning said catalytic configuration, is present; and/or- decreasing (s440) the temperature of said catalytic configuration from a prevailing temperature (TSred) above said predetermined temperature value (Te) to a temperature below said predetermined temperature value so as to impair the performance of said catalytic configuration in case of sulphur, poisoning said catalytic configuration, is present; and- determining s(450) one cause out of a set of causes on the basis of said course of said NO-conversion ratio thus determined.The invention relates also to a computer program product comprising program code (P) for acomputer (200; 210; 500) for implementing a method according to the invention. Theinvention relates also to a system for determining a cause for impaired performance of acatalytic configuration and a motor vehicle (100) equipped with the system.