Catalytic converters are used in internal combustion engines to reduce harmful carbon monoxide emissions. They also break down nitrogen oxides to nitrogen and oxygen and hydrocarbons into water and carbon dioxide. Catalytic converters accomplish these tasks by causing chemical reactions with the exhaust emissions as they flow through. The catalyst used most often in converters are platinum, rhodium or palladium, as these are very reactive metals. The core of the converter is often ceramic or stainless steel. They are formed like a honeycomb to increase the surface area, which increases the effectiveness of the catalyst. Many problems can arise with catalytic converters, which can affect how the engine runs.