System och förfarande för att diagnostisera en sensor hos ett avgasreningssystem

The invention relates to a method to diagnose an NOx-sensor (222), arranged downstream of an arrangement comprising an SCR-catalyst (270) in a catalytic exhaust treatment system to purify exhausts from an exhaust generating system, comprising a combustion engine (231), comprising the steps: - to continuously determine an NOx-level (NOx2) with said NOx-sensor (222), - to perform at least one action with the objective of increasing the SCR-catalyst arrangement's conversion degree with respect to NOx and/or to reduce said NOx-level (NOx2) downstream of said SCR-catalyst (270), in order to determine an off-set indication for said NOx-sensor (222), wherein said off-set indication is treated as a minimum NOx-level value, which is the result of actions performed, wherein at least one action consists of at least one of the actions: - controlling (s410) the operation of said combustion engine (231) with the objective of reducing the NOx-level (NOx1) in exhausts; - controlling (s420) the exhaust generating system with the objective of increasing said SCR- catalyst's (270) temperature (Tmeas; Tmod); - increasing (s430) the supply of reductant to said SC-catalyst arrangement; and - reducing (s440) a mass flow of said exhausts to said SCR-catalyst arrangement. The invention also relates to a computer program product comprising program code (P) for a computer (200; 210) to implement a method according to the invention. The invention also relates to a system at an exhaust treatment system and a motor vehicle (100) equipped with said system.