A method pertaining to an SCR system whereby reducing agent is supplied to an exhaust flow upstream of an SCR catalyst, the content of NO x in the exhaust gases is measured upstream and downstream of the SCR catalyst and any presence of reducing agent crystals in the SCR catalyst is detected, characterised by the steps of - shutting off the supply of reducing agent to the exhaust flow at a first time so that the SCR catalyst is substantially emptied of vaporised reducing agent, - measuring NO x contents upstream and downstream of the SCR catalyst at a later second time corresponding to a first temperature of the SCR catalyst at which reducing agent crystals vaporise, - comparing NO x content measured upstream of the SCR catalyst with NO x content measured downstream of the SCR catalyst and, if the difference between NO x content measured upstream of the SCR catalyst and NO x content measured downstream of the SCR catalyst exceeds a certain value, taking this as an indication of possible presence of reducing agent crystals in the SCR catalyst, - measuring NO x contents upstream of the SCR catalyst and downstream of the SCR catalyst at a third time corresponding to a second temperature of the SCR catalyst at which reducing agent crystals do not vaporise, - comparing NO x content measured upstream of the SCR catalyst with NO x content measured downstream of the SCR catalyst and, if the difference between NO x content measured upstream of the SCR catalyst and NO x content measured downstream of the SCR catalyst is below a certain value, taking it that the NO x contents are correctly measured, and - concluding that reducing agent crystals are present, provided that the NO x contents are correctly measured and the presence of reducing agent crystals in the SCR catalyst has been indicated. - shutting off the supply of reducing agent to the exhaust flow at a first time so that the SCR catalyst is substantially emptied of vaporised reducing agent, - measuring NO x contents upstream and downstream of the SCR catalyst at a later second time corresponding to a first temperature of the SCR catalyst at which reducing agent crystals vaporise, - comparing NO x content measured upstream of the SCR catalyst with NO x content measured downstream of the SCR catalyst and, if the difference between NO x content measured upstream of the SCR catalyst and NO x content measured downstream of the SCR catalyst exceeds a certain value, taking this as an indication of possible presence of reducing agent crystals in the SCR catalyst, - measuring NO x contents upstream of the SCR catalyst and downstream of the SCR catalyst at a third time corresponding to a second temperature of the SCR catalyst at which reducing agent crystals do not vaporise, - comparing NO x content measured upstream of the SCR catalyst with NO x content measured downstream of the SCR catalyst and, if the difference between NO x content measured upstream of the SCR catalyst and NO x content measured downstream of the SCR catalyst is below a certain value, taking it that the NO x contents are correctly measured, and - concluding that reducing agent crystals are present, provided that the NO x contents are correctly measured and the presence of reducing agent crystals in the SCR catalyst has been indicated.