Got P0420 and P0430 codes, suggesting the catalytic converter needs to be replaced. Everything I have read suggests the wave form after the cat ( sensor 2) should be a delayed version of before the cat ( sensor 1 ). Is this correct?

What I am seeing is below. This figure is from an OBD software tool. I didn't believe so I double checked by actually connecting to the pins in the ECM as described by the Toyota service manual. I didn't have a scope, but I saw a constant 3.3 volts on sensor 1 and variable voltage on sensor 2 between between 0 and 1 volt. So that seems to conform these graphs.

I'm a novice at auto repair but have some engineering in my past. Can anyone explain this? Is my cat bad? Should I replace it? Thanks from an old retired guy trying to save a buck.

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  • Was this taken at idle? A solid 3.3v @ idle on the B1/2 AFR sensors is normal on Toyota's. What do the rear o2's do when on the highway? I'd say the cats are bad. Barring an exhaust leak or out of spec fuel trim.
    – Ben
    Commented Dec 4, 2017 at 22:50
  • About 2000RPM, engine was warm. Thanks. On the road, about 35 mph it's about the same, sensor 1 moves a little more. I haven't measured things at highway speed, should I? Thanks. Commented Dec 4, 2017 at 23:21
  • Probably not necessary if this was taken while driving. May be worth it just to have the data. I don't see any TSBs concerning the ECM calibration either. Can you go into mode 6 and pull the test results to see how bad the failure was?
    – Ben
    Commented Dec 4, 2017 at 23:27
  • I added the "mode 6" page to the question above. Let me know if this is not what you were looking for. Thanks! Commented Dec 5, 2017 at 0:17

1 Answer 1


The readings for the front the sensor 1, sensors look normal. These are air-fuel ratio sensors not standard oxygen sensors. The 3.3 volts is a PCM generated representative voltage. It is not the actual output reading. That would be in milliamps. It will stay relatively steady as long as the mixture does not change rapidly.

The rear sensors are standard oxygen sensors. These readings, with the swings up and down from .2 to .8 volts, are indicative of failed catalysts. Good catalysts would be showing steady voltage readings.


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