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Old 09-12-2019, 11:12 AM
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O2 sensor data: Help me understand

Hi there, I'm trying to diagnose P0420. I know it's probably the cats but I want to eliminate other lower-cost possibilities first. I just pulled the data from my O2 sensors and it looks like this:

O2S2 Bank 2 looks suspicious to me. Does anyone have any insight into what this data is telling me? Thank you!

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Old 09-12-2019, 11:34 AM   #2
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Imgur blocks sites like this that create a lot of traffic. (You can probably see them because they're already cached for you locally. I cut-and-pasted the image URLs into a new browser window.)

High voltage is rich, low voltage is lean. Is that open loop (cold start) or closed loop (cats already up to operating temp)? I'm guessing open loop since S2B1 is so rich (the computer is trying to heat the cats). But yeah you'd expect both S2 banks to be doing approximately the same thing like you see with S1.

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Old 09-12-2019, 11:42 AM
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Ah, okay, good to know about Imgur. Here's a link to both images:

The start of the data is open loop and it's closed loop toward the end.

I *think* what this is telling me is bank 1 O2S2 (downstream sensor) is reading too rich, too high of voltage, and therefore the bank 1 cat isn't doing its job.

In which case it's the cats and the O2 sensors seem to be reading fine. Does this sound right?
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Old 09-12-2019, 02:51 PM   #4
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Usually P0420 is a code that just means you need to new cats. Only a few things you can check before you spend the money.

1) you need to check that the o2 sensors are reporting reasonable numbers. Aka, can they report rich and can they report lean. To report rich, run high revs or drive on a road and floor it. This will cause the engine put as much fuel through the injectors and cause a rich condition. You should see all four o2 sensors go high or closer to 1v. If not you may have a bad O2 sensor. To get the o2 sensors to go lean, pull a vacuum hose off at idle. You should see the o2s report low or closer to 0v. If not you may have an o2 sensor problem. Make sure you plug your vac line in again.

2) Another thing to check is map your b1 sensors (for P0420) and b2 sensors together (for P0430). Then drive and compare the graphs. With a functioning cat converter, the sensor 2 or post cat sensor shouldn't have the cycling of high/low like the front sensor. The cat should remove quite a bit of the oscillating. If the S2 looks almost identical to the S1, it means the cat is not doing much. Hence the P0420 code.

The graphs you show are tough to interpret because you have too much time on the screen so the data is jammed together.
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Old 09-13-2019, 07:58 AM
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Great insight, thank you. I'll run some more tests to ensure the o2 sensors are behaving reasonably, and compare the sensors together.
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oxygen sensor , p0420

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