code po420,new cat and o2 sensor

I have a 1998 Chevy Venture with a code po420 set. I have replaced both oxygen sensors and the catalytic converter. The code was cleared by the muffler shop when they replaced the converter and stayed off for about one
month. It came back this week. Does anyone have any ideas on what could be wrong? I know this code is telling me about the converter not operating at peak efficiency. Thanks, Terry
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Are there any other codes, or just P0420?

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Steve Mackie wrote:

changed, could this affect the cat? The van runs great, no missing, hesitation and gets good gas mileage. My wife informed me last night the SES light had gone off. I am not very confident it will stay off, however. Thanks, Terry
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Search google, there are other people that had this problem. If you replaced the cat and the sensor, and then the code came back a month later maybe there is something messing up the sensor or cat. Coolant, fuel, oil, etc. Could be another bad sensor causing a rich fuel mixture which will eat cats in a hurry. I would have the vehicle checked by the dealer.
Steve

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It is amazing how many people just throw parts at a problem instead of analyzing it properly. A good technician with an OBD2 tester can check most of the sensors and performance of the engine so the correct parts are replaced. Stop guessing and have it checked....

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