P0420 troubleshooting sensors

Got the dreaded P0420 " Efficiency Of Catalyst System Is Lower Than Limit - Bank 1" on my 2010 Tacoma V6. I read several of the related articles on the site and they recommend checking O2 sensors before and after the catalytic converters. I have an A30M Xtool scanner, and can see the O2 sensor 2s appear to be in the 100-900mV range, but what are the before cat sensors called? Is it the "AF Lambda Bank 1 Sensor 1"? If so, is the test the same? Anyone know what their readings should be?

P0420 troubleshooting:
https://parts.olathetoyota.com/p0420-code-toyota#:~:text=Error code P0420 - or Catalyst,catalytic converter and exhaust system.

O2 Sensor troubleshooting:
https://parts.olathetoyota.com/blog/failing-toyota-oxygen-sensor

My readings after 30 minute engine warmup:




Full diag data

 

If it helps, the below shows current value, max, and min value. Does this look like it's operating normally? Or indicating my cat are actually bad?

 

I am not well versed on the values of O2 sensors and their cousins, air/fuel ratio (AFR) sensors. But, I have read that lambda readings are just a different scale - it is a numerical representation of the stoichiometric ratio, one being the ideal ratio. A value under one is rich and over one is lean.

 

Your upstream sensors would be AFS Voltage, you can't really look at static data. You need to graph the sensor voltages.


It's very rare that I see an AFR or O2 sensor as the cause for a P0420, an exhaust leak near or upstream of the AFR or O2 sensor can cause it but don't see that very often either. Most of the time it's a catalyst that is no longer functioning.

To understand why the code is setting you have to understand how the PCM monitors catalyst efficiency and the voltage thresholds for an O2 sensor.
Because a normally functioning converter stores oxygen this allows the PCM to monitor their function using the AFR and O2 sensor.

There are 2 methods used to monitor the catalytic converter function:

Downstream O2 switch rate: The PCM compares the rich/lean switch rate of the upstream and downstream sensors under certain driving conditions. (older vehicles for the most part used this method and were relatively easy to "fool" with various methods)

Oxygen storage capacity: The PCM will deliberately force the air/fuel ratio to a rich or lean state under certain driving conditions and measure the length of time between when the upstream sensor switches and when the downstream sensor switches. (almost all modern vehicles use this method or a combination of both methods and are harder to "fool")


A standard O2 sensor generates a 0-1v signal based on oxygen in the exhaust, because they have a very narrow operating window essentially anything above 0.5V (500mV) is rich (<14.7:1) and below 0.5V (500mV) is lean (>14.7:1) although usually the "dead zone" threshold for a lean/rich "switch" is around 0.45V (450mV) - 0.55V (550mV)


The way we check converters and sensor operation with a scan tool is graphing the AFR and O2 sensor data.
It's kinda hard to explain what good and bad data looks like as far as sensor operation but typically with a good functioning cat at operating temp the downstream sensor should stay relatively constant at around 0.7V (700mV).


Here's some known good captures, you can see how the AFR (Upstream) sensors "switch" very rapidly and the O2 (Downstream) sensors stay relatively constant.

First one is an accel and decel event and the 2nd is a steady cruise.