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Message: <blockquote data-quote="everythingIwanttoknow" data-source="post: 1592095" data-attributes="member: 620622">Hi everyoneMy toyota matrix filled with Diesel in gas tank by mistake, and I took it to Midas,they clean the gas tank and blow the engine, and they changed the spark plug, and added the injector cleaner liquid. But two days later,the check engine light is on. I took it to autozone and scan it, the code came out was P0420 (catalyst system efficency below threshold(bank1) )I have on idea what this means. could someone please help me.Is this the O2 sensor? and which one? where does it locate?Thank you</blockquote>