A good catalytic convertor would be above .650 volts. below that number you would set a P0420 catalytic below threholds
Here a good catalytic convertor, here it above .650 volts. The RPM is being raised and it drop , as the RPM is held steady it goes back up above .650 volts
Here is a 02 Sensor 2 that set a P0420. This is a flase P0420. This 02 sensor is faulty and set a false P0420. Also if this wave is seen on a upstream 02 sensor or Sensor 1 it would set a P0171 Fuel trim lean as this is a faulty 02 sensor . It is a shorted 02 .
Here another false P0420 Even though the voltage is above .650 volts, you can see that the 02 is shorting out. This ECM set the correct code indicating a 02 heater circuit code.
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