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Re: P0420 Error: Unauthorized Catalytic Converter or Oxygen Sensor
1. yes an aftermarket cat can cause that code, if not efficent enough. a cat and o2 sensor should be changed at the same time. cats really only go bad with oil or a too rich condition for a long time. did you do a 90 k service? have the plugs been changed. computer doesn't read the o2 sensor until it goes into closed loop so if it happens when the car starts, you may have another problem. u can unplug the 02 sensors but the car will only run the base/wot fuel map it won't adjust to rich or lean conditions and the cel will be on. you may be able to return the o2 just get it on a scanner and the check the voltage output. most likely you will need a new cat because that may have been contaminated because you didn't change the o2 or have a recent tune up.
drew
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