The present invention relates to a method for restricting work produced in a combustion chamber, wherein a substance resulting from combustion is reduced by supplying additive comprising a reagent to the exhaust gas stream. The method comprising: determining if a quality of said additive, estimated by means of a first sensor, being subjected to said additive, is below a first quality; determining if signals from a second sensor, being subjected to the exhaust gas stream, indicate insufficient reduction of said at least one substance; determining if a correction of the supply of additive has reached an upper limit; and restricting work produced by combustion only when: said estimated quality of said additive is below said first quality, said correction has reached said upper limit, and said measurement in the exhaust gas stream indicates insufficient reduction.