How To Check 02 Sensor With Torque App

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It's possible the O₂ sensor you replaced was only telling you the truth.
Given the mileage, and assuming you've never replaced the upstream A/F sensor with OEM/Denso, this may be another piece in the puzzle.
At 250k, it is very likely that the cause of the failed catalyst monitor test is a bad catalytic converter. If the engine uses a lot of oil, then the cat will stop working (die) due to oil contamination.
My P0420 checklist is;

NO exhaust leaks from the cylinder head to 18" past the downstream sensor.
No misfires or excessive oil use.
Good fuel trim numbers (LTFT 0 +/- 10%, STFT usually less than 1% variation at steady engine operation such as idle)
A/F and O2 sensors can respond to rich and lean conditions. (A/F about 3.3V @ idle, 2.0V with propane enrichment, 5.0V @ decel fuel cut; downstream O2 over 0.9V with propane, and less than 0.1V decel). Any replacement sensor should be Toyota or Denso from a reputable source.

IF those are all pass then it gets a cat. If it's not an OE cat then the code could return "shortly". If it uses oil the code could return shortly.

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Does anyone have any experience with $06 mode testing?
$06 Fail ( see pic posted above) Details:
MID:$21 TID:$a9
Catalyst Monitor Bank 1
Min 0.02 Max 9.994
Current 0.013

I get that my Gen2 is out of specification for this one test.

Do we know what the "MID $21" test is?
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Same $06 testing gives upstream sensor passing marks.
Joined:

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I found a reference to the $21 Monitor ID in the Prius Service Manual, p. 259. See pic attached. Additionally, this reference explains that this monitor and test are described as "Oxygen storage capacity of catalyst bank 1".
Interesting but I do not know how this gets me closer to diagnosing/fixing the problem.
Here was my result a somewhere slightly over 150,000 miles:

The test value is still above the minimum, but not by a lot. I should be mindful about saving up some pennies for a catalytic converter. At some future point my number will drop below the minimum, a P0420 will be reported, and it will be time for me to spend them.
I'm not sure I understand your question. The catalyst monitor consists of ONE test, that the ecu runs to determine catalytic converter efficiency. That test is MID: $21 TID: $a9, and the results of the last test performed are stored in Mode $06 data (where you found them). If the test passes, (in this case the result is above the minimum), then no codes are stored and you go about your life as normal. If the test fails (below the minimum), then the ecu stores the P0420 code as "pending". If the test fails twice in a row, the code "matures" to an active code and the MIL is turned on.
So your problem is that the test failed- MID: $21 TID: $a9 is just telling you that.

The test used is the Oxygen Storage Capacity test- OSC. A catalytic converter works by exposing exhaust gases to precious metals (such as platinum or palladium) which causes a reaction to convert pollutants (CO, HC, and NOx) into H2O, CO2, and N2. The part where CO and HC react requires oxygen- more than might be available. So the converter is coated with another material (cerium I believe) which temporarily holds onto excess oxygen then releases it when needed. OSC measures the converter's ability to do just that- if it can't store oxygen, then it pretty much can't do anything at all. Testing OSC is easy to accomplish and provides "good enough" indication of cat function.

The test runs after the car has been driven enough to get the engine and converter hot- then (usually at idle) the ecu forces the fuel mixture rich until the A/F and O2 sensors indicate rich (all the O2 in the cat is used up), Then it drives the mixture lean and measures how much time passes from when the upstream A/F sensor goes lean until the downstream O2 follows. A healthy cat will "grab onto" the O2 from the exhaust and there will be a lag before the O2 sensor reports lean (can be over 10 seconds). If it has less capacity, the O2 sensor will switch sooner- at around 2-3 seconds or less it will set a code. The specifics have a bunch more variables so all we get is some engineering gibberish in Mode $06 with a pass or fail.

As far as diagnosing, I use my checklist above. Exhaust leaks allow ambient O2 to be sucked in which screws up the OSC. The engine has to run correctly to provide good "feed gases" to the cat for it to function (no misfires, good fuel trim, good A/F and O2 sensor operation). Just because the upstream A/F sensor passes doesn't mean that it works 100%. It can deteriorate quite a bit before it will fail enough to throw a code. Sometimes I have gotten a marginal cat failure to pass (at least for a short time) by replacing the A/F. Not every case but some. And if you are going to replace the cat, then you want a new A/F anyway to give the cat its best shot at a long life.

Here's the cat monitor on my 06 at 170k
the test numbers and values used will change depending on if it's the "initial" test after a code clear, or after a fail result, or maybe based on phases of the moon (aka, engineering gibberish)
Quinton_Hall Junior Member

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What Bluetooth adapter or you using?
Quinton_Hall Junior Member

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Thank you. Last one I tried was the more expensive vepeek ble.
I've order a harness to try techstream also.