What is the minimum time rate of change (absolute value) of the rear o2 sensor signal voltage that will trigger a p0420 dtc? This is probably vehicle specific, but a ball park figure would help. Thanks

  • Toyota doesn’t give specifics on this. If the rear o2 is swinging up and down at steady speed with normal afr sensor operation the cat is bad. Normal ho2s operation at steady speed should be around 650mV. I’m not even sure crosscounts are used as part pf the monitor strategy. I believe it’s, command the afr sensor when the monitor is enabled. Check for specific changes in voltage.
    – Ben
    Apr 13, 2018 at 2:07

1 Answer 1


There are as many different strategies for deteriming catalyst failure as there are vehicle manufacturers. All the formulas are propritary, even a deep dive into SAE technical papers on the subject will reveal little about the details of the varius strategies. It is easier to say what the post catalyst sensor waveform of a working catalyst looks like than what a failed one looks like. A working catalyst waveform will not vary at all in response to ossiclations of the pre-cat sensor when tested at steady rpm. That is not to say that the rear sensor does not change over time. The systems often switch from rich to lean and back over time. This is strategy for fuel mileage improvement and catalyst health.

The rear sensor should show as a straight line with no bumps or ripples. Any ripple no matter how small a voltage change that is in sync with the pre-cat sensor will be viewed as likely catalyst failure.

Reference this scanner graph from a failed catalyst on a Honda

Failed catalyst scan data

Notice that the after cat sensor (green line) is low on the graph indicating an overall lean mixture and that the voltage amplitude is relativly small. It is in perfect sync with the pre-cat sensor (red line).

Now consider this capture taken after the catalyst was replaced with a new one.

New catalyst

The overall mixture is still lean but now the rear sensor shows no cycling behavior.

  • Thanks for the information. I was working on a filter, and I was trying to find a suitable bandwith so as to avoid both P0420 and P0136. It's working well now.
    – Ray Sutton
    Apr 15, 2018 at 3:06

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