P0420 Mazda - would that be the downstream o2 sensor that might need to be replaced

Question

With the p0420 code, is it a upstream or downstream o2 sensor i would have to replace?

Answer 1

P0420 Catalytic below threshold Bank 1

With a P0420, check the operation of the 02 sensor the front and the rear, The front 02 sensor is the engine management or the upstream and the rear is the catalytic monitor sensor or the downstream 02 sensor. You would check the operation of the rear 02 sensor.

 A good catalytic convertor , would show on the rear 02 sensor reading of being above 0.650 volts, if your are below that voltage for the rear 02 sensor you need a catalytic convertor