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Air/fuel ratio sensor testing thread

  • Tyler
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28 Jan 2018 16:09 #17336 by Tyler
Replied by Tyler on topic Air/fuel ratio sensor testing thread

ecwurban wrote: So really wideband sensors aren't very complex. What's complex about them is trying to find information on how individual systems interpret their data and how they represent it to you! How every manufacturer represents them differently and gives you access to them differently!


Truth. :( I find it very tough to broadly generalize about how to interpret air/fuel ratio scan data, because no two makes do it the same way. Even within makes, there's differences between years and models.

As usual, the best way I've found to get familiar is to hook up to known good cars and take them for a drive. :cheer:

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07 Mar 2018 22:07 - 07 Mar 2018 22:08 #18705 by chief eaglebear
Replied by chief eaglebear on topic Air/fuel ratio sensor testing thread
Excellent info article I am gonna revue this in my studies of the af sensor your insight is excellent
Last edit: 07 Mar 2018 22:08 by chief eaglebear.
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10 Mar 2018 09:06 #18776 by Tyler
Replied by Tyler on topic Air/fuel ratio sensor testing thread
Thanks! I really gotta get around to updating with more makes. :blush:

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06 May 2018 06:55 - 06 May 2018 06:58 #20484 by SailorBob
Replied by SailorBob on topic Air/fuel ratio sensor testing thread
So is this the catch all thread for O2 sensor questions?

One thing I'm interested in is using the downstream O2 for A/F diagnosis. For example, I assume that if the car is running lean, i.e. positive fuel trims, then the downstream O2 should read rich, and visa versa, if the car is running rich, i.e. negative fuel trims, then the downstream should be pegged lean? This assumes the upstream O2 is oscillating more or less normally, and both O2s and the cat are known good.

Does that make sense? Is it true in practice?

I ask, because sometimes all you've got is your scope because your scan tool won't talk to that specific car for whatever reason.
Last edit: 06 May 2018 06:58 by SailorBob.

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06 May 2018 09:16 - 06 May 2018 09:28 #20485 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic Air/fuel ratio sensor testing thread
Dosen't work that way , if the fueling is still within closed loop control range ie the fuel trims are not maxed or min'd out both upstream and downstream sensor output will look pretty normal although snap throttle might show something different . But if the fuel trims are maxed/min'd out and the engine computer can't add or take away any more fuel then both upstream and down stream will show lean or rich reflecting the conditions in the cylinder.

The next stage of investigation would be to check if the sensors are actually responding to real conditions and stimulate the sensors by introducing a vacuum leak or adding propane.
The other useful method is to watch what how the sensors compare when responding to the accelerator pedal.

"There's always a catch ---- Catch OBD2 ."


Last edit: 06 May 2018 09:28 by Andy.MacFadyen.

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06 May 2018 11:30 #20492 by SailorBob
Replied by SailorBob on topic Air/fuel ratio sensor testing thread
OK, so then I'm completely confused. I put propane in the intake for about ten seconds, the upstream responded immediately, but after ten seconds I didn't see any down stream response and stopped. DS stayed steady at about 150mv. So I ran the engine at about 3.5k for 60 seconds and then at 2k for another 30 seconds and the DS gradually rose to about 860mv and stayed there even after letting off the gas. I then created a large vacuum leak and the DS gradually went to zero after 2.5 seconds, then after plugging the vacuum leak it went back up to 650mv after about 23 seconds of idling. I can see occasional misfires in the US waveform during this whole episode. I can upload the video if you're interested ( didn't record the propane part ). This is all on a warmed up engine.

Don't know how to interpret this.

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07 May 2018 11:32 - 07 May 2018 11:42 #20524 by graywave
Replied by graywave on topic Air/fuel ratio sensor testing thread
I have been caught off guard with downstreams and their readings. What I found on some rare vehicles, the computer doesn't always turn on the heaters at idle. While your driving the heater will turn on (regardless if its PWM or Straight voltage) and off when the computer wants to check the CAT.

I found this on a couple cars and one was a toyota, I think it was a 2008. Possibly a corolla or Camry. Don't quote me on the year and model but it was a toyota.

I almost thought it had a bad computer since the downstream O2 was reading very lean and was not reacting to propane. Though reving the engine for extended period of time warmed up the O2 enough for it to function normal without the heater until I let off the gas and it immediately seem to slowly loose its reaction. After checking the heater circuit I found there was no control to the O2's heater but the heater in the DS O2 was good. Only while driving did I see heater circuit control on the lab scope from the PCM under specific load conditions.

There was NO codes for O2 heater circuit or Catalytic converter but was a p0171 which seemed to go away after doing intake manifold gaskets. I was trying to verify the upstream AFR via the DS readings but couldn't.

Oxygen sensors need to hit specific temperatures to function correctly. If the heater is active and pulling current I would be looking for small exhaust leaks with a stethoscope (open tube end).

If the engine is lean, you may or may not see it in the ds O2 sensor depending on how lean the engine is. If its maxed lean (max positive correction) you might see it stay lean, may not switch up and down. If you ever see a DS o2 switching, you may have an issue with the CAT.

How I understand narrowband o2 sensors is the "switching" comes from the computer adding fuel when the reading is lean and taking away fuel when the reading is rich which causes the switching we see. Since these o2 sensor only register within a very narrow AFR window the exhaust mixture needs to be "close" (within limits) to stoich for it to start switching. If its only being used to monitor the cat, if the cat is working you may never see it switch unless the cat is flooded with O2 and the reading is then lean and suddenly the mixture goes rich.

Confirm what it's not, and fix what it is!
Last edit: 07 May 2018 11:42 by graywave.
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20 Aug 2018 02:04 #22739 by Flatrater
Replied by Flatrater on topic Air/fuel ratio sensor testing thread
Some random thoughts on this topic. Very random, I've been coding all day :-)

The biggest difficulty I see with AF sensors is how the output is reported in the data stream. But even that I don't see as an obstacle when technicians are taught to understand fuel control properly. I think a real issue here is that most technicians are not able to be dedicated to only driveability. They often have to service not just the whole vehicle, but chain saws, tractors, and whatever the heck comes in the door.

90+% of an AF sensors circuitry is in the ECM, not the sensor.

Zirconia O2 sensors do more than just switch above and below stoich. 800mv is not the same as 900mv for example. You can see this by spending time looking at downstream sensor output and watching NOx numbers. A slightly biased rich mixture produces much less NOx without a mileage penalty. A zirconia sensor can do that.

Much of what has been requoted all over the web concerning zirconia and AF sensors is really old information. AF and O2 sensors were used for many years before much research was done concerning their operation in the field over time. Significant changes came along after the research, some of which is fairly recent.

AF sensors are comparatively slow and their use requires a different fuel control strategy, which at the end of the day, means managing the catalyst. GM, for example, is not "behind" because they primarily use zirconia sensors upstream. It is by choice and it is cheaper when you can handle the required software.

A zirconia sensor can do a pretty good job of targeting mixtures above or below stoich when needed, and have for some time.

AF sensors are not used to run lean air fuel ratios for any length of time in the US, doing so would deactivate the catalyst and require a NOx trap.

AF sensors "switch" (in a sense) or "are switched" just like zirconia sensors. Most do not see this because they are not zooming in properly. While a perfectly stoich mixture makes the catalyst more efficient, one that moves slightly higher and lower than stoich will widen the range of catalyst efficiency.

The equivalence ratio pid found on the generic side of the scan tool is actually lambda. The engineer in charge of that (inside information) made a mistake as equivalence ratio is the opposite of lambda. It is the fuel-air ratio, not the air-fuel ratio.

If you're wondering why more indepth information isn't generally available, I can tell you why. It costs $$, research is really tough, and no one (technicians) wants to pay for it.
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04 Sep 2018 14:20 #23077 by TheTechWhisperer
Replied by TheTechWhisperer on topic Air/fuel ratio sensor testing thread
Wow, this is a really nice piece you wrote here Tyler. I don't get on this forum as often as I should, I'm going to have to change that! This whole topic is so incredibly murky in the industry. I really think that for those of us who have only learned/been taught the "ins and outs" of just a couple of these specific AFR systems (which is probably most of us), it's almost been a disadvantage. I think I may have been better off only learning the basic concept :lol:

For example, I have only learned 2 of these variations thoroughly. The problem is that you unknowingly get a false sense of confidence, and tailor your expectations to the premise that "they all kind of work the same". In theory, they do... BUT, when it comes to interpreting enhanced PIDs, they really do have some crucial differences! AND that's even without the added twist of plugging in 2 scanners and getting different data, haha (not going to open that can of worms).

One thing is certain in this business: Maintaining the careful balance between confidence and humility is the recipe for success. The second that balance tilts too far one way, a car and/or your paycheck will remind you.

"You will always find the greatest fulfillment in life when you are operating in the gifts God gave you"- Dad

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05 Sep 2018 06:27 #23092 by Noah
Replied by Noah on topic Air/fuel ratio sensor testing thread

TheTechWhisperer wrote:
One thing is certain in this business: Maintaining the careful balance between confidence and humility is the recipe for success. The second that balance tilts too far one way, a car and/or your paycheck will remind you.


Well put.

"Learn, apply, repeat."

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05 Sep 2018 09:21 #23095 by Tyler
Replied by Tyler on topic Air/fuel ratio sensor testing thread

Flatrater wrote: If you're wondering why more indepth information isn't generally available, I can tell you why. It costs $$, research is really tough, and no one (technicians) wants to pay for it.


I will. :ohmy: But I couldn't find any training that told me what I really wanted to know. :(

Wow, this is a really nice piece you wrote here Tyler. I don't get on this forum as often as I should, I'm going to have to change that!


Thanks! Please feel free to add any insights you have on the A/F sensors you're familar with, if you want. This thread is far from comprehensive. :lol:

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14 Mar 2019 07:53 #27807 by ScannerDanner
Replied by ScannerDanner on topic Air/fuel ratio sensor testing thread
Holy crap, I think I need to be here more, what a phenomenal write up. Thanks Tyler!! (I'm only 2 years late)

Don't be a parts changer!

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14 Mar 2019 11:27 #27808 by Tyler
Replied by Tyler on topic Air/fuel ratio sensor testing thread
:lol: No worries, Paul! I really need to make some corrections and add what I've learned about VW and Ford sensors. There's so many variations out there. :ohmy:

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15 Apr 2019 02:01 #28694 by chasencai
Replied by chasencai on topic Air/fuel ratio sensor testing thread
very nice write-up.Thank you for you long post to explain

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