On the 2006 Mitsubishi Lancer, The P0421 is a fairly straight forward code pointing to a failing Warm-up Catalytic Converter. The Oxygen Sensors measure the amount of emissions in the exhaust going into, and then out of the converter. When they see that the converter is not “cleaning” the exhaust enough to meet specs, the Converter is deemed “below threshold” and the P0421 is set. Make sure there are no current cylinder misfires. Make sure there are no leaks in the exhaust system. Which, if there were, you should be able to hear. Sounding like a repeating “tick” or “puff” from the exhaust. Usually more predominate when the engine is cold. Make sure to include the exhaust manifold when checking for leaks/cracks. If these check ok, then most commonly, the Converter is “on it’s way out” and will need replacement in the near future. Keep in mind, you will need a replacement converter that meets specs. Those “universal” or “discount” converters sold at some after market parts stores do not meet specs, and will not cure a P0421. On this vehicle, the “correct” Warm-up Catalytic Converter is built into and part of the exhaust manifold.
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