Apparatus (100) for controlling an aftertreatment system of an internal combustion engine (101), a system comprising an apparatus, a vehicle comprising a system and a method (1000) of controlling injection in an internal combustion engine (101) are disclosed. The apparatus comprises a processing means (102) configured to receive a first signal from a first temperature sensing means (103) indicative of a first temperature of exhaust gases outputted from an internal combustion engine (101) at a first location upstream of a first exhaust system component (104) configured to provide a passage for exhaust gases. The processing means is also configured to receive a second signal from a flow rate sensing means (105) indicative of a flow rate of the exhaust gases outputted from the engine (101) and calculate an approximated value at least from the first signal and the second signal. The approximated value is indicative of a second temperature of exhaust gases at a location downstream of the first exhaust system component (104). The processing means is further configured to provide an output signal to control the after treatment system, in dependence of the calculated approximated value.