Lexus Troubleshooting Questions? Ask a Lexus Expert
Unfortunately, the P0420 is a fairly straight forward code, and a fairly common problem. This code points to a failing Catalytic Converter (for Bank1 - this vehicle has 2 Converters). 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 P0420 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 P0420. If you found this answer helpful, please help me by leaving a rating using the "stars" at the top of the page or click 'accept'.Rating or accepting my answer does not cost you extra. It allows me to be credited for this Q&A.
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The tires being mismatched could set the traction light, however, if you are getting a "P0420" - this is a failing Catalytic Converter. Very well known fail item. Very straight forward code.
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