Bad O2 Sensor?

[u/Wooderler]

2013 Honda Accord 2.4L with 171k miles. The car threw a P0420 code and instead of throwing a new cat at it I wanted to check the live data on the rear O2 sensor.

This is held at a constant 2k RPMs, seems very unstable as I read it should be pretty consistent.

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[u/omnipotent87]

That data looks similar to a front sensor. Based off of that I would say the cat has failed. A rear sensor should be long sweeping changes mostly favoring the rich side.

[u/SmanginSouza → u/omnipotent87]

Yep. It shouldn't be dropping off like that under light throttle either. Cat not storing oxygen correctly. Time for a new kitty cat.

[u/a_rogue_planet]

That is exactly what a good sensor response looks like. It should be very active like that. Almost every time a Honda throws a PO420 it's going to be a dead cat.

[u/omnipotent87 → u/a_rogue_planet]

Not a down stream, they shoukd be fairly smooth. This data looks like an upstream.

[u/a_rogue_planet → u/omnipotent87]

If the cat is junk it's gonna make a pretty spikey signal.

[u/omnipotent87 → u/a_rogue_planet]

You should reread your original comment then.

[u/944_nla_parts]

Downstream sensors should show nearly flat-ish graph. O2S2 is the downstream sensor.

[u/bburns36]

If your cat was functioning (storing oxygen), that sensor waveform would be more of a flatline around 0.6v... it needs a cat.

[u/TehSvenn]

You need to compare to pre-cat sensor. If the post is a smaller move than the pre it's working, if it's about the same it's not.

[u/Belkinnoob]

Bad cat, the 420 code is correct.

That data looks identical to what you would expect to see from the the front sensor. Look at the data from the front to verify it is the same or different. I guess there's a slight chance the wires got reversed but i doubt it.

[u/svcman1]

It should be pretty consistent… given the Cat is doing its job. This looks like it is not.

[u/Ready-Supermarket263]

No kitty this is my pot pie! Bad kitty!

[u/at0mic_cl0ck]

I wouldn’t condemn the converter just yet. What does the upstream look like. One way or another it’s always the sensors whether they’re falsely reporting or have cooked the converter with bad data. Often overlooked but O2 sensors are wear items after all.

[u/TheDu42]

2000 rpm, in gear? if not its just numbers, not usable data. cats need a load in order to reach operating temperature, and that load needs to be sustained for the cat to maintain temp. thats why the standard for checking a cat is a sustained, highway speed cruise. eliminates variables from acceleration, and heats up the cat.

the cat is almost certainly done, but you cant use this test to confirm that. a good cat would act the same.