A method for controlling an exhaust after-treatment system connected to a diesel engine via a gas passage and having a diesel oxidation catalyst (DOC) and a “diesel-exhaust-fluid” (DEF) injector arranged downstream of the DOC and upstream of a selective catalytic reduction catalyst (SCR) includes detecting a flow of exhaust gas emitted by the engine into the gas passage. The method also includes detecting a level of nitrogen oxides (NOx) in the exhaust gas downstream of the SCR. The method additionally includes activating the DEF injector for a period of time to reduce the level of NOx to a predetermined NOx value. Furthermore, the method includes regulating an injection of fuel upstream of the DOC to clean the DEF injector via a stream of superheated exhaust gas if the period of time used to reduce the level of NOx to the predetermined NOx value is greater than a predetermined threshold value.