A vehicle comprising a diesel engine and an exhaust system comprising an oxidation catalyst that does not comprise rhodium and a downstream diesel particulate filter or a catalysed soot filter emissions control device for treating an exhaust gas from a diesel engine and means for heating the DPF or the CSF to a regeneration temperature, which is an injector for injecting combustible hydrocarbon into the exhaust gas downstream of the diesel engine, which oxidation catalyst comprises: a flow-through monolith substrate; a first washcoat region comprising a first washcoat layer disposed on the substrate, wherein the first washcoat region comprises a first platinum group metal, which is a combination of platinum and palladium and a first support material, which first support material does not comprise cerium oxide; a second washcoat region adjacent to the first washcoat region, wherein the second washcoat region comprises a second platinum group metal, which is a combination of platinum and palladium, and a second support material; a third washcoat region disposed on the substrate, wherein the third washcoat region comprises a third platinum group metal selected from the group consisting of platinum and a combination of platinum and palladium and a third support material, wherein the third washcoat region is adjacent to the second washcoat region, the arrangement being such that exhaust gas entering the oxidation catalyst comes into contact with the second washcoat region before the third washcoat region, and wherein the first washcoat region comprises a part of an upstream washcoat layer, the second washcoat region comprises a part of the upstream washcoat layer and a part of a downstream washcoat layer, and the third washcoat region comprises a part of the downstream washcoat layer, wherein the total loading of the second PGM is greater than the total loading of both the first PGM and the third PGM. a flow-through monolith substrate; a first washcoat region comprising a first washcoat layer disposed on the substrate, wherein the first washcoat region comprises a first platinum group metal, which is a combination of platinum and palladium and a first support material, which first support material does not comprise cerium oxide; a second washcoat region adjacent to the first washcoat region, wherein the second washcoat region comprises a second platinum group metal, which is a combination of platinum and palladium, and a second support material; a third washcoat region disposed on the substrate, wherein the third washcoat region comprises a third platinum group metal selected from the group consisting of platinum and a combination of platinum and palladium and a third support material, wherein the third washcoat region is adjacent to the second washcoat region, the arrangement being such that exhaust gas entering the oxidation catalyst comes into contact with the second washcoat region before the third washcoat region, and wherein the first washcoat region comprises a part of an upstream washcoat layer, the second washcoat region comprises a part of the upstream washcoat layer and a part of a downstream washcoat layer, and the third washcoat region comprises a part of the downstream washcoat layer, wherein the total loading of the second PGM is greater than the total loading of both the first PGM and the third PGM.