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Elite Member
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Join Date: Feb 2011
Location: Bend, OR
Posts: 9,902
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Elite Member
Join Date: Feb 2011
Location: Bend, OR
Posts: 9,902
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Ah the P0420... one of those annoying little codes that we dread. First thing is first, a p0420 is not a problem in itself, it is a symptom of something else. That is not to say that your catalytic converters couldn't be bad, but just replacing them if they have failed will not fix what actually caused them to fail in the first place.
A large majority of P0420's seem to be caused by faulty/failing O2 Sensors and AFRs. Normally the vehicle will set a code for a failing sensor if it gets out of the accepted ranges for what it considers "normal operating condition." But this leaves enough room for the sensor to fall into a sort of gray area that can cause a catalytic converter monitor to fail and thus set the code. Unfortuntely our vehicles have both types of sensors and that makes it difficult to accurately diagnose which one is causing the problem. O2 sensors can be easily tested with a DSO hooked up to the sensor and monitoring the waveform's shape, amplitude, and reaction time. Unfortunetly, I am unaware of any reliable way to test a Air Fuel Ratio Sensor because of how differently they operate compared to a O2 Sensor.
Normally we reccomend replacement pre and post cat oxygen sensors as a repair for the code, particularly if there is no other code set along with it and the catalytic converter is in good condition. The reality is though, that without doing an intrusive test you cannot acurately measure the efficiency of a catalytic converter.
Best of luck, I have thought about replacing my AFR & O2 Sensors on my own 4Runner, wich is about $200 in parts alone.
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