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P0420: Catalyst System Efficiency Below Threshold (Bank 1).
Your vehicle's Computer Monitors the efficiency of your catalytic convert through the use of 2 oxygen sensors. There is on before the cat, which is supposed to send a fluctuating signal to the car's computer. The one behind the cat is supposed to have a steady signal that changes very little. This code is set when both sensors begin to send similar reading to the computer and it can mean that the cat is no longer disposing of harmful exhaust gases or one of the sensors is no longer functioning.
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