Method of controlling operation of an exhaust gas treatment apparatus

An engine apparatus (500, fig 1) comprises an internal combustion engine (510, fig 1) and an exhaust gas treatment apparatus (550, fig 1) having a diesel oxidation catalyst (520, fig 1) upstream of a second aftertreatment module (530, fig 1) such as a diesel particulate filter (DPF) or a selective catalytic reduction (SCR) catalyst. Where a numerical indication of an efficiency of the second aftertreatment module falls below a first threshold value, a first limit on engine speed is implemented. When an engine safe condition is reached, a primary cleaning process (regeneration) is carried out comprising injecting fuel into the engine after an engine combustion event such that the fuel passes through the engine without combusting for the fuel to combust in the diesel oxidation catalyst to increase exhaust gas temperature in the diesel oxidation catalyst by a target amount, and removing the first limit on engine speed and targeting an engine speed set point, which is at a higher speed than the first limit on engine speed.