A method of controlling operation of an engine, the engine comprising: a combustion unit having an exhaust conduit and one or more combustion cylinders each having a piston, a fuel injector, an intake valve and an exhaust valve leading to the exhaust conduit; an exhaust gas treatment apparatus configured to receive fluid from the exhaust conduit and comprising a diesel oxidation catalyst module; and a controller configured to receive a temperature data value indicative of a temperature of gas within the diesel oxidation catalyst module. The method comprises: injecting fuel from the fuel injector into each of the one or more combustion cylinders in a main injection event to initiate via compression ignition a primary combustion event that drives the piston. The method further comprises using the controller to compare the temperature data value relative to a first temperature threshold value and a second temperature threshold value.