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Q: Could a Exhaust leak cause P0420? on 1999 Toyota 4Runner

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I recently have been getting the P0420 code in my 4runner with CA emissions. Which means I have two catalytic converters. I took it into a shop where they took the voltage readings for the o2 sensors and they said it looked good. I was under the hood playing with the throttle while passing "Seafoam" through to clean the engine and noticed that some of that seafoam smoke was coming from in between exhaust pipes. The leak is at the connection right after the headers so I'm assuming it is just bad gaskets. I get the CEL P0420 code usually under hard accelerations, rainy weather, or stop and go driving. Unusual thing is that when I took it for a road trip from Portland to San Diego, and also from Portland to Seattle, the code never came on! Please help!!
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Here is a ton of great info about the P0420 code, what causes it to come on and how to diagnose it: http://repairpal.com/OBD-II-Code-P0420-Toyota
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