A vehicle comprising a compression ignition engine provided with engine management means and having a catalyst for exhaust gas aftertreatment, wherein the engine management means is configured, when in use, to detect idle conditions and upon determining that idle conditions exist, stops the engine entirely, wherein the catalyst comprises: a honeycomb substrate monolith coated with a catalytic washcoat comprising one or more precious metal, which catalytic washcoat being arranged between a first, upstream washcoat zone and a second downstream washcoat zone, and wherein a washcoat layer in the first, upstream washcoat zone is substantially contiguous with a washcoat layer in the second, downstream washcoat zone; and wherein: a thermal mass in the first washcoat zone is different from a thermal mass in the second washcoat zone, such that the thermal mass in the first, upstream washcoat zone is less than the thermal mass in the second, downstream washcoat zone, wherein the different thermal mass in the second, downstream washcoat zone relative to the first, upstream washcoat zone is provided by a thicker washcoat layer than is used in the first, upstream washcoat zone, wherein the thicker washcoat layer is provided by a washcoat loading of from 244 to 610 gL -1 (4 to 10 gin -3) and a relatively thinner washcoat layer in the first, upstream washcoat zone has a washcoat loading of from 61 to 214 gL -1 (1 to 3.5 gin -3); a total precious metal loading, measured as unit weight of precious metal per unit volume of washcoat, in the first, upstream washcoat zone is greater than a total precious metal loading in the second, downstream washcoat zone; the first, upstream washcoat zone comprises 60 to 80% of the total precious metal loading of the honeycomb substrate monolith; and the honeycomb substrate monolith has a total length, wherein the first, upstream washcoat zone is defined at an upstream end by an inlet end of the honeycomb substrate monolith and at a downstream end by point between 20% and 40% of the total length of the substrate monolith measured from the inlet end; and wherein the one or more precious metal is platinum, palladium, a mixture of platinum and palladium or a combination of palladium and gold. a honeycomb substrate monolith coated with a catalytic washcoat comprising one or more precious metal, which catalytic washcoat being arranged between a first, upstream washcoat zone and a second downstream washcoat zone, and wherein a washcoat layer in the first, upstream washcoat zone is substantially contiguous with a washcoat layer in the second, downstream washcoat zone; and wherein: a thermal mass in the first washcoat zone is different from a thermal mass in the second washcoat zone, such that the thermal mass in the first, upstream washcoat zone is less than the thermal mass in the second, downstream washcoat zone, wherein the different thermal mass in the second, downstream washcoat zone relative to the first, upstream washcoat zone is provided by a thicker washcoat layer than is used in the first, upstream washcoat zone, wherein the thicker washcoat layer is provided by a washcoat loading of from 244 to 610 gL -1 (4 to 10 gin -3) and a relatively thinner washcoat layer in the first, upstream washcoat zone has a washcoat loading of from 61 to 214 gL -1 (1 to 3.5 gin -3); a total precious metal loading, measured as unit weight of precious metal per unit volume of washcoat, in the first, upstream washcoat zone is greater than a total precious metal loading in the second, downstream washcoat zone; the first, upstream washcoat zone comprises 60 to 80% of the total precious metal loading of the honeycomb substrate monolith; and the honeycomb substrate monolith has a total length, wherein the first, upstream washcoat zone is defined at an upstream end by an inlet end of the honeycomb substrate monolith and at a downstream end by point between 20% and 40% of the total length of the substrate monolith measured from the inlet end; and a thermal mass in the first washcoat zone is different from a thermal mass in the second washcoat zone, such that the thermal mass in the first, upstream washcoat zone is less than the thermal mass in the second, downstream washcoat zone, wherein the different thermal mass in the second, downstream washcoat zone relative to the first, upstream washcoat zone is provided by a thicker washcoat layer than is used in the first, upstream washcoat zone, wherein the thicker washcoat layer is provided by a washcoat loading of from 244 to 610 gL -1 (4 to 10 gin -3) and a relatively thinner washcoat layer in the first, upstream washcoat zone has a washcoat loading of from 61 to 214 gL -1 (1 to 3.5 gin -3); a total precious metal loading, measured as unit weight of precious metal per unit volume of washcoat, in the first, upstream washcoat zone is greater than a total precious metal loading in the second, downstream washcoat zone; the first, upstream washcoat zone comprises 60 to 80% of the total precious metal loading of the honeycomb substrate monolith; and the honeycomb substrate monolith has a total length, wherein the first, upstream washcoat zone is defined at an upstream end by an inlet end of the honeycomb substrate monolith and at a downstream end by point between 20% and 40% of the total length of the substrate monolith measured from the inlet end; and wherein the one or more precious metal is platinum, palladium, a mixture of platinum and palladium or a combination of palladium and gold.