2010 crv. P0420 popped up. Connected elm scanner. It showed dead bank one air fuel ratio sensor. Replaced sensor. .4 volts coming from sensor. P0420 still comes on. Elm scanner still shows a dead sensor even though my multimeter shows almost 1/2 volt steady at idle. Reset pcm. Light stays off for a while and comes back on. I am lost trying to figure this one out.

 

P0420 states that the catalytic converter is operating at below efficiency
It means there is something wrong in that system

O2 sensor 1 is the upstream sensor just after the exhaust
That is the air fuel ratio O2 sensor
The fuel is constantly being adjusted and so the O2 sensor should be signaling the changes
Showing a constant voltage is not correct.
Also, one needs a graphing multimeter or oscilloscope to read the output from an O2 sensor
Regular meters will not show the changing signal
Depress the gas pedal and the reading should change

Downstream sensor just after the catalytic converter
That is the cat efficiency O2 sensor
The O2 sensor is reporting the cat is not working at its proper efficiency
Sometimes it is the O2 sensor that is faulty
Sometimes it is the catalytic converter that is faulty
Some people change the O2 sensor in the hopes that the cheaper part is faulty
Depress the gas pedal and the readings should change

Hope this helps.