Abstract: A method for on-board diagnosis of an oxidation catalyst in an exhaust-gas system of an internal combustion engine of a vehicle having an engine control device and a temperature sensor in the exhaust gas system upstream from a downstream side of said oxidation catalyst. The method includes operating the internal combustion engine is in an overrun mode, and injecting a quantity of fuel upstream from the oxidation catalyst. The change of temperature values over time is measured by the temperature sensor, and at least one physical quantity that characterizes the heat release in said oxidation catalyst is calculated on the basis of said amount of injected fuel. The method further includes ascertaining whether the calculated quantity falls within a set of values, and, if so, declaring that said oxidation catalyst is considered to be defective.