Dear Doctor: I own a 2002 Mercedes-Benz S-Class with 97,000 miles. It needs a left-side catalytic converter. The factory replacement is $2,600 for one side. Why did this fail on an $80,000 car with less than 100,000 miles? My mechanic suggested an aftermarket replacement for under $1,000. What are the pitfalls to an aftermarket converter? It carries a five-year warranty. Martin
Dear Martin: Catalytic converters fail for many reasons, such as mechanical engine faults, oil burning, engine misfire, engine temperature too cold or hot, too much or too little fuel, ignition timing, EGR valve operation, plus other factors. Most factory original catalytic converters carry an eight-year or 80,000-mile warranty. It is not often I would recommend an aftermarket catalytic converter for a vehicle. Most inexpensive aftermarket converters fail after only 40,000 miles. A high-quality aftermarket replacement in this case is what I would recommend.