A short cyclic thermal treatment process in an engine aftertreatment system of a diesel engine in a vehicle, which may be able to recover and/or retain NOx conversion performance of an engine aftertreatment system by keeping the system clean from unintended pollution like accumulated urea deposits. The thermal treatment process is controlled by a process controller, arranged to carry out a long cyclic cleaning process at a first time interval, at a first exhaust gas temperature higher than the operational temperature in regeneration mode; combined with a short cyclic thermal treatment process at a second time interval, at a second exhaust gas temperature higher than the operational temperature in non-regeneration mode; wherein the second elevated exhaust gas temperature is lower than the first temperature.