A system for reducing ammonia in exhaust gas generated from a lean burn internal combustion engine includes an oxidation catalyst, a selective reduction catalyst (SCR), a cooling unit, and a three-way catalyst. Exhaust gas generated by the engine passes through the oxidation catalyst to oxidize carbon monoxide from the exhaust gas to form carbon dioxide. Nitrous oxide (NOx) compounds in the exhaust gas are reduced in the SCR to form nitrogen and water. The exhaust gas is then cooled in a cooling unit and then passed over the three-way catalyst. The three-way catalyst causes ammonia in the cooled exhaust stream to react to form less harmful compounds, such as nitrogen and water.