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Discussion Starter · #1 ·
2013 manual trans, 130k miles.

I am getting a slightly rough idle when the car is warm and lost about 1 or 2mpg. I was thinking about replacing the upstream O2 sensers. No lights, no codes. Is there a way to check the functionality of the sensers? I have cleaned the MAP senser and the throttle body. I plan on replacing the plugs in the near future. Those O2 sensers are kind of expensive. Is it advisable to replace before getting a code?

Toyota parts lists show three upstream O2 sensers. A left, a right and one which is labeled both left and right. All have different part numbers. Am I right assuming that the one can go in either location. Is there any advantage getting the left and right specific parts. The whole thing odd. Why would toyota stock two extra parts? Rockauto makes no distinction. They just sell one part for either location.

I doubt that there is an answer to the second issue but any thoughts would be appreciated.

John
 

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Those forward "O2" sensors are AFR (air/fuel ratio) sensors, which is why they are more expensive. They can go bad, but normally have a code if they do. You would need a scan tool to monitor them to see if they are getting sluggish. Spark plugs are due at 120k I think. Change those first and see what happens. Make sure you use the OEM plugs as well.

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Spark plugs are much cheaper to replace and yes, go OEM. Also look for vacuum leaks, chewed wiring to the injectors and coils and perhaps replace the PCV valve if you don't know how long since it's been replaced. Then get a scan tool or even a Bluetooth scan tool and use your phone as the display.
 

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Discussion Starter · #5 ·
I have scanned it and there were no codes. I did replace the pcv valve. Old one was probably OK, you could hear the valve rattle. How to tell if it the O2 senser is "sluggish". I have a tool which will monitor values, hopefully exhaust O2 levels, real time. What should I look for?
 

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Toyota AFR sensors work at around 3.3 volts. Above that is lean and below is rich. If you quickly pump the gas pedal with engine running, you should see a change to rich. Create a big enough vacuum leak and you should see it go lean.

FJtest would be the guy to ask about these.

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Here is a video of a mechanic troubleshooting AFR problem. Start around the 13:00 minute mark. Good info.

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The A/F and the O2 sensors tend to wear after about 100k miles and engine running issues begin without throwing codes.

Replace them, after 130k miles its time.
 

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I have scanned it and there were no codes. I did replace the pcv valve. Old one was probably OK, you could hear the valve rattle. How to tell if it the O2 senser is "sluggish". I have a tool which will monitor values, hopefully exhaust O2 levels, real time. What should I look for?
I think your platinum tipped spark plugs are supposed be changed at 100,000 miles.
 

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I just replaced both of my AFR sensors at 240,000 and will be watching my gas mileage on a upcoming trip to the White Rim trail in Utah. Hoping for some improvement 🤞
Closely monitor not only fuel consumption (precise quantity fuel burned from receipt divided by exact distance traveled from odometer), but also subtle driveability factors like throttle response,smoothness of transition from closed throttle to open throttle, idle speed and smoothness immediately after a cold start, etc.

I don't think I've ever heard of a vehicle that ran flawlessly with 240K miles on the fuel-air sensors. Are you absolutely certain that both fuel-air sensors had 240K miles on them?
 

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I just replaced both of my AFR sensors at 240,000 and will be watching my gas mileage on a upcoming trip to the White Rim trail in Utah. Hoping for some improvement 🤞
Post up if your mpg improves. Mine is at 171K no lights or anything but my mileage is super crap right now. I know my drivers side front brake is dragging a bit. I`m going to order new front calipers and I think im going to get new wheel bearing assemblies while im in there. :)
 

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Closely monitor not only fuel consumption (precise quantity fuel burned from receipt divided by exact distance traveled from odometer), but also subtle driveability factors like throttle response,smoothness of transition from closed throttle to open throttle, idle speed and smoothness immediately after a cold start, etc.

I don't think I've ever heard of a vehicle that ran flawlessly with 240K miles on the fuel-air sensors. Are you absolutely certain that both fuel-air sensors had 240K miles on them?
Absolutely, the only work I’ve had at a dealer are alignments. 3/4ths of those miles are highway though.
 

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Absolutely, the only work I’ve had at a dealer are alignments. 3/4ths of those miles are highway though.
Do you have any OBD app available (like Torque Pro, etc.) that allow you to watch 'commanded' and 'measured' fuel-air ratios before and after the sensor replacement? Yours is such an unusual case that it would be worth the effort to see the before and after data. F-A sensors can degrade in multiple ways, probably the most benign is just slow response.

I know on my 60K mile '14, commanded and measured A-F ratios are always very close to 14.7:1, and the response time is nearly instantaneous.
 

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Discussion Starter · #17 ·
Still not completely clear. Is there a difference between the left and the right sensors. Toyota online parts shows Left and Right as different part numbers. Rockauto shows Walker, Denso, Ultra Power and NTK as one sensor fits either location. It could be something as simple as the length of the leads is different.

In another thread someone stated that Denso was the OEM. Can someone confirm that?
 

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Denso is the OEM supplier for all Toyota fuel-air and oxygen sensors.
Denso shows one fuel-air (upstream) sensor part number (234-9144) for both Bank-1 and Bank-2 positions.
The two oxygen sensors (downstream) have different part numbers.
 

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Discussion Starter · #19 ·
This could be a source of my confusion. The online Toyota parts list calls them both O2 sensors and the parts diagram is not clear which way is upstream. I was advised that the upstream ones were generally the problem and was just going to change them out as a maintenance item. Instead, I need to hook up the OBD2 tool and monitor them for while to see if I really have an issue. I will start with that video from bridem11.

I have a clear direction now.

Thanks everyone for the help.
 

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What is your "OBD2 tool"?

Does it allow you to read the actual voltage output of the fuel-air sensors, real time, like the latest version of Torque Pro does?

That will allow you to monitor the sensor outputs under various load conditions and verify that the voltage levels are at least close to where they should be, that both Bank-1 and Bank-2 sensors are providing very similar data, and the that the response time is near instantaneous.

No value in replacing $300 worth of sensors if their outputs are still within spec, and whatever you see in their outputs might point to other issues that are affecting fuel economy and idle smoothness.

However, before spending any time looking at the F-A sensors, have you cleaned the MAF sensor, and are the spark plugs well under the recommended replacement interval?
 
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