The catalytic converter is located upstream of the muffler, close to the engine, and it changes environmentally damaging pollutants in the exhaust stream into more benign gases. Under extreme heat, precious metals in a honeycomb structure catalyze the chemical reactions; they make today’s vehicles dramatically cleaner than older vehicles. Catalytic converters can be both causes and victims of check engine lights. A failing “cat” can trigger a check engine light and/or produce an unpleasant exhaust odor, but converter failure is often caused by abnormal combustion and rough engine operation, sometimes accompanied by a blinking check engine light. Because of its precious metals content, converters are expensive to replace and often targeted by thieves.