Yeah, me too. Losing thePRIMARY (front) O2 sensor causes it to go into open-loop. The rear O2 sensor doesn't close the control loop at all. It's just monitoring the cat.

Considering the secondary O2 sensor is located downstream of most of the catalyst, it's not evenmeasuring the proper thing to close the fuel-control loop.

 

Reviving an old tread. I have the same message on my 2000 Accord 4 cylinder. It says below threshold on bank 1. Is that the front sensor or rear?

 

If you got dtc P0420, the problem is not likely the secondary O2 sensor but the catalytic converter.

If you have a scanner you can check the secondary (behind converter) voltage trace for faulty converter operation (frequent 1-2 hz, voltage swings from 0.1 -0.9 volts. If that pattern is present the converter has failed.

Or you can simply replace the secondary O2 sensor and see what happens,

good luck

 

Thanks Tex! I do have a scanner, I'll check for the voltage fluctuation. I noticed by looking at other sites that the front O2 sensor fluctuates but the rear is supposed to remain steady.