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 (DPF) or a catalysed soot filter (CSF) 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 (PGM), 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 (PGM), 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 (PGM) 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 (PGM), 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 (PGM), 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 (PGM) 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. <EOS>