Patent ID: 8099946

Claim:
A diagnostics method performed during operation on board a vehicle and for an oxidation catalyst, for the oxidation of NO into NO 2 , arranged in a vehicle with an internal combustion engine which, during operation, emits exhaust gases to an exhaust gas aftertreatment system containing at least the oxidation catalyst and at least one of a NO x -reducing catalyst and a particulate filter arranged downstream of the oxidation catalyst, comprising: adding a predetermined quantity of a reduction agent to the exhaust gas aftertreatment system during at least one predetermined time interval upstream of the oxidation catalyst; measuring one of NO x content and NO 2 content downstream of the oxidation catalyst; making a first measurement of and recording a first value for one of the NO x content and NO 2 content at a point in time before the reduction agent is supplied to the exhaust gas aftertreatment system; making a second measurement of and recording a second value for one of the NO x content and NO 2 content at a point in time during the time interval when the reduction agent is supplied to the exhaust gas aftertreatment system, where the supply of reduction agent temporarily poisons the oxidation catalyst so that a quantity of NO 2 produced is negligible; comparing the first and second values and obtaining an indication of impaired function of the oxidation catalyst when a difference between the first and second values is less than a predetermined first value.