Postby smj999smj » Fri Sep 21, 2012 10:21 pm
There is no mileage interval to change the catalytic converters. You replace them when them become inefficient (in which it will throw a three way catalyst trouble code if it's an upstream converter) or when they become restrictive, which will usually result in poor acceleration, potential setting of miscellaneous trouble codes and can be checked with an exhaust backpressure tester. High NOx emissions are caused by excessive heat or excessive pressure in the combustion chamber. I remember taking a training class when I was with Nissan and the trainer telling us something like 90% of emission failures are due to high NOx emissions and 90% of those vehicles can be fixed by a top-end engine cleaner.