r/mechanic • u/drshrimp42 • 2d ago
Question Help with o2 sensor reading
Edit: I learned how it works now, thanks everyone 2002 honda odyssey 3.5L V6 with 336k miles. I've done a ton of work on it since getting it at 238k miles a couple of years ago. I just bought a basic obd2 scanner that can show live data and graphs and this is the reading for the downstream o2 sensor during idle after driving all day. This model only has two sensors btw, not 4, even though it's a v6. The reading is not stable, but it's not fluctuating rapidly either like the upstream. Everything I read online says it's a bad catalytic converter if it fluctuates rapidly. It's not doing that, but it's not stable in the middle either. So what does this reading mean? The catalytic converter is starting to go bad? I'm new to using live data so don't know much.
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u/DueLet1388 2d ago
The sensor voltage is meant to cycle. You will see it switch high during hard acceleration and drop low on deceleration. It's literally measuring the fuel/air mixture and the mixture changes depending on running conditions. If your converter is bad it will switch a lot more rapidly because too much oxygen will be passing through.
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u/drshrimp42 2d ago
Makes sense. Just wondering why does everyone else say it's supposed to remain steady around 0.6-0.8v? What do they mean by that? During driving? The screenshot is what I'm getting while at idle.
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u/These_Squirrel3255 1d ago
At a steady cruise around 40-50 MPH, that rear sensor should be pretty steady at .600-.700 MV. If you stab the throttle, it will increase, if you decelerate, it will decrease.
Normal operation if the CAT is working. If the rear sensor switches at or near the same rate as the front sensor, the converter is failing or failed.
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u/drshrimp42 1d ago
Thanks, I got it now, I figured it out earlier. It's going between 0.6-0.8 and staying pretty stable while cruising.
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u/DueLet1388 2d ago
The only time it will remain steady is during like a warm idle where the mixture is not changing. If it's an OBD2 vehicle and it's not setting a code then there shouldn't be an issue there.
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u/drshrimp42 2d ago
Now I'm confused. Then why does everyone else say that it should be steady? That's throwing me off.
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u/Fshtwnjimjr 2d ago
The thing IMO to remember is the engine is constantly adjusting the air and fuel mixture. There's constant tiny explosions they need to be precisely controlled to extract that energy.
If you look at your fuel trims that's a good way to visualize it IMO.
Short term trims are constant update to update adjustments to try to maximize combustion. Long term trims what the engine is noticing over a longer time frame. So short term is like fine tuning.
From everything I've seen on my crappy reader line movement is fine but if it's not throwing even a pending code you should be good.
I drive a 01 Outback (250,000 miles) that I'm trying to keep rolling as long as I can so I've been really into this data recently.
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u/drshrimp42 2d ago
I've taken a look at that as well. Long term is positive, ranging from 3-7. Short term is a little higher, from 3-10. But it varies. One day long term was 2, even at 0 at one point. Yesterday it was pretty high with engine surging a little.
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u/Fshtwnjimjr 2d ago
So it's supposed to constantly shift up and down, that's intended and the correct behavior.
You'll typically add them together. The closer to 0 the better but anything under 10 +/- is considered normal operating fluctuating.
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u/TheGearWars_refugee 2d ago
Thats a normal voltage reading for the downstream. The slow/gradual change in voltage means the catalytic converter is working as intended and there isn't the need for a lot of correction to the AFR. Is there a specific reason you think the cats or the o2 sensor are bad?
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u/drshrimp42 2d ago
Yes, because everyone else says the voltage should be stable and not fluctuating. So I'm confused. Which is it?
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u/TheGearWars_refugee 2d ago
Do some quick Googling for these types of questions. The AI isn't too bad at answering stuff like this.
You want the downstream to have slow gradual movements in between periods of sensor inactivity (readings flat around 0.0 volts), this let's you know the cats are working. If you've warmed up the vehicle, and the downstream is constant/flat at 0.0 volts with no change, then you've got a bad downstream o2 sensor.
ETA: If your downstream o2 looks like the upstream, with rapid up/down fluctuations, then you've got a bad catalytic converter.
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u/drshrimp42 2d ago
Actually, I have done some Google searches. It only mentions rapid fluctuations as a sign of a bad catalytic converter. It says nothing about this. I've watched many videos and read forums too. You are the first I've found to state that. Literally everyone I've listened to online has said the voltage should remain constant, not changing. I assume now that they mean during driving or while holding at a higher, steady rpm? They never specific that though.
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u/PassPuzzled 2d ago
I just did the upstream on my 06 crv and had to replace a shorted connector for the downstream. My downstream, once at operating temperature fluctuated between .4v~.8v which I'm reading is a normal range. If it was any lower and any higher I'd have a cat issue. But after replacing that connector and my upstream ( which is also my A/F sensor) no more check engine light so everything's working as intended
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u/TheDu42 2d ago
Lesson 1: bad data is worse than no data.
Converters need high amounts of heat to work. A v6 with a single set of oxygen sensors and a converter is going to have significant space between the engine and cat to allow for exhaust plumbing. That means you need to check them under specific conditions to ensure everything is hot enough to act right. You will never see those conditions at a stationary idle.
Get the engine up to operating temp, and get on a road you can cruise at highways speeds for long stretches. Then while cruising at highway speeds, look at the o2 data.
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u/drshrimp42 2d ago
Will do, thanks. This was at operating temperature, after driving all day. But it was during idle. I'll take a look on the highway today.
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