Method of controlling operation of an exhaust gas treatment apparatus

A method of controlling an engine assembly comprising an internal combustion engine and an exhaust gas treatment apparatus which comprises a diesel oxidation catalyst (DOC) module and a downstream selective catalytic reduction (SCR) module. The method comprises comparing a numerical indication of an efficiency of the SCR module with a first threshold efficiency 120. If the efficiency remains above the first threshold, a time-initiated SCR cleaning event is initiated 140 after a time interval has elapsed 130. If the efficiency falls below the first threshold, a threshold-initiated SCR cleaning event is initiated 160 without waiting until the time interval has elapsed. Each SCR cleaning event comprises injecting fuel into the engine after combustion such that the fuel combusts in the DOC module to increase the exhaust gas temperature. The method further comprises determining whether a set of criteria is met, which includes determining that, following the threshold-initiated SCR cleaning event, SCR efficiency exceeds a second threshold efficiency 170. If the set of criteria is met, an error warning associated with high sulphur fuel is triggered 180.