How long are the cat. Converters known to last on 2003 lebarons

Hello, there is no "rule" or "expectation" on how long a catalytic converter can last. If the engine is running properly the entire time, and there is not excessive quantities of fuel running through the engine and then exhaust system, your converters may be just fine for a long time to come. However, there's no measurement method, nor is there a replacement interval. My best advice would be to maintain proper engine performance health so as to avoid catalytic converter damage caused by unnecessary overheating due to excessive fuel in the exhaust stream.