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Air/Fuel & O2 sensor mini write-up

44K views 60 replies 27 participants last post by  CafeRoaster 
#1 ·
Just did both air-fuel sensors and both O2 sensors on my 2008 today. I didn't find much in write-ups for this work so I figured it was either super easy and nobody needed it or not many have done it. Either way, here is what I did and a problem I ran into. Nothing earth shattering but maybe it will help someone considering doing on their own.

Symptoms leading up to the replacement: I was having some off idle hesitation up to about 2500 RPM or about 35MPH but it wasn't 100% of the time and nothing I tried could make it happen or make it stop. I posted a thread in the engine performance section about the issues I was having and got a lot of good advice. I cleaned the throttle body and bought a blue tooth OBD2 reader to get a better look at what was going on. I didn't have any codes but something wasn't right. After cleaning the throttle body (off engine) and taking a test drive with the OBD2 reader reporting on the Air/Fuel ratio commanded and the Air/Fuel ratio monitored nothing jumped out at me right away but I did notice that sometimes there was a pretty big Delta between the two and they were not really closely matched all the time. I was still running the OEM sensors and in the other thread had learned I was well past what was an expected life for all of them. I decided to go ahead and swap them all out in effort to restore the lost performance.

Specailty tools I used:
OBD2 Bluetooth
Torque Pro App
Lisle Dual Drive O2 offset wrench
Lisle O2 thread chase
I used Denso A/F ($108) and Denso O2 ($48) sensors from Rock Auto

Normal tools I used:
I didn’t keep a comprehensive list, I replaced all 4 sensors from underneath and I did use several 1/2" extensions with the Lisle wrench on the A/F sensors, 3' extension with U joint on the passenger side exhaust bracket upper 14mm bolts (from engine compartment), 14mm ratcheting box end on the driver’s side exhaust bracket upper bolts, 22mm box end wrench for the O2 sensors and 3/4" ratcheting box end wrench on the Lisle thread chase. The wiring harness plugs are set free by way of a standard screw driver with a narrow head and about a 3-4" length.

Drivers side A/F sensor:
The FSM alludes to removing a bracket with 3 bolts and by the picture in the FSM I wasn't sure what it was. Once under the driver’s side I saw what was in the way - a big bracket with two bolts on the engine block side and a single bolt on an exhaust coupling. Removing these bolts took a lot of effort and a cheater bar, which had either a bunch of torque from factory or had just taken a hard set over the years/miles.

Once the bracket was out of the way I was able to use a short/narrow standard screw driver to release the wiring harness from the sensor pig tail. I used the O2 wrench with 1/2" drive extension as a handle and a short cheater bar to get the A/F sensor to break free. Once the sensor was broken free it unscrewed by hand pretty easy. I ran the chase in/out the threads with little to no resistance. I coated the new Denso A/F sensor threads with their provided copper colored anti-seize and ran the sensor into the threads by hand until lightly seated. Once it was seated I used the Lisle O2 wrench to seat the sensor with a reasonable amount of torque.

Passenger side A/F sensor : The exhaust bracket was a PITA to remove. I could only reach the two upper bolts with a 3' extension and a U joint. Even then it took a cheater bar to pop each one free. The bottom bolt came out without issue. Getting the wiring harness free took the narrow standard screw driver but by the time I had my arm up where it needed to be I couldn’t see the plug so I had to do it by feel. Took me a couple tries to unplug the sensor. Removing the A/F took the Lisle O2 wrench but this time with a 3/8 socket wrench in the top side of the O2 wrench and a cheater bar on the socket wrench handle. Once it popped free, it was easy to un screw by hand. I ran the O2 thread chase in/out with little to no resistance, applied the copper color anti seize and installed the A/F sensor.

Drivers side O2 sensor : The sensor unplugs same as the A/F sensor with the short narrow standard screw driver. I used a 22mm 12 point box end to unscrew the sensor (the pig tail on the sensor fits through the box). The O2 sensor was tough to break free and just as tough to unscrew to remove. A couple times I stopped to make sure I wasn’t accidentally tightening it by accident. I even pondered if it might be reverse threaded for some reason. In the end it came out with a lot of effort. The threads on the old sensor looked like crap and I was worried it might have galled the threads and I was right. The threads in the exhaust pipe were in bad shape. It took a couple starts to get the O2 thread chase started square but once started I would turn it in about 1/4 turn and then back it off 1/2 turn or more and run it back in another 1/4. I pulled it out to clean and relube several times. It took a while but I eventually got the threads re-cut and cleaned up. I was able to install the new sensor with the copper anti seize without drama.

Passenger side O2 sensor: After all the drama on the driver’s side I was a little hesitant to pull this sensor out. I was able to unplug the pig tail and pop the sensor free without drama. This one unscrewed by hand once popped free and the thread chase ran in/out with little resistance. I coated the threads of the new sensor with the copper anti-seize and installed it by hand and set the final torque with the 22mm box end wrench before plugging the pig tail back in.

I re connected the negative side of the battery and started the FJ. It fired right up to a pretty fast idle, like it would on a very cold day (it was about 75F today). The initial idle seemed to have a little roughness to it but there were no apparent exhaust leaks or dash warning lights. I shut off the engine, plugged in the OBD2 scanner and started the engine. I ran through the scanner readings and everything seemed to be ok with no codes. I took a couple runs around the block and everything ran great. I took a longer test drive after cleaning up and its very noticeably better. Throttle response is back to what I remember and everything is smooth at idle. The initial idle roughness I felt at first start went away fast and everything settled in during this longer test drive.

I am really happy I bought the O2 thread chase! Both Lisle tools were great quality (forged) and I am pleased with both the pruchases. I almost skipped the chase but I would have been SOL today without it. I have no idea what happened with the threads in the drivers side O2 sensor, maybe I somehow messed it up while unscrewing it or maybe it got jacked up when it was installed on the assembly line. I'll probably never know but I am damn happy I had that thread chase and was able to recover from that situation.
 
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#5 ·
What is thread chase? Never heard of it. Thank-you.
I linked to the thread chase in the section for the specialty tools I used. A thread chase is like a thread tap but it's not designed to cut new threads rather its used to clean up existing threads and fix minor issues in the threads. In my case, I ended up using it more like a tap and it felt like it was cutting new threads quite a bit. Having some basic knowledge and experience with taps helped me to not over use the tool and risk breaking it off. The technique I used was to run the clean and lubed chase in about 1/4 turn and then back off at least a half turn or more. Backing off allows the chase to clean the cutting threads into the debris channels. After a full turn or two you want to back the chase all the way out and clean it off and re-lube the threads. It is slow going but you dont want to force it or go too fast - too much heat/friction will make things go from bad to worse quickly.

Normally when using a thread chase you will just find a rough spot or two in the threads and that is what the chase helps to cut/reform. The other three sensor holes had very little need for the chase other than a spot or two that was easy to turn through with little effort. I mostly used the chase to verify the threads were in good shape and to help apply a coat of the anti-seize to the threads since that is what I lubed up the chase with for the three good holes.

Considering the price was so cheap and it saved the day for me, I am very happy with the purchase. I can't say that everyone will need it for the sensor, heck I may end up with the only FJ that has the problem.
 
#6 ·
Just a quick update for anyone that finds this thread while doing research for their own troubleshooting.

After changing all 4 O2 sensors and resetting the ECU the overall performance did improve significantly. Even with the improvements, I could still detect some minor hesitation in off idle part throttle acceleration at times. It was not always consistent and same as before I could not make it happen or make it go away when it did happen. I still wasn't getting any codes thrown so I wasn't sure where the issue was occurring. Overall everything felt like it was about 90-95% back to normal but something still wasn't quite right at times.

I decided to replace the MAF sensor with a new Denso unit from Rock Auto ($65). I had previously cleaned the MAF several times (all parts, the match head and the recessed little wires) but I was willing to accept that at 188K maybe the MAF just wasn't up to snuff any longer. I replaced the MAF and reset the computer a couple nights ago and things are now back to 100% normal. I no longer have any hesitations or flat spots in power delivery while accelerating from off idle with part throttle.

The first 1/4 tank of gas after the MAF replacement was showing signs of poorer than normal MPG but I am chalking that up to the computer settling in and me making several big stabs at the throttle while driving in effort to see how things responded. I have topped off the tank and reset the trip meter to make a new MPG calculation and so far its all looking good. I will keep the thread updated if this ends up not being the end of the issues but so far its all now back to normal and no negative signs or symptoms.

So far the biggest learning experience for me has been that the sensors can be degraded enough that its really obvious something isn't right but not degraded enough to throw a code. I had always believed that the computer would complain soon as things drifted out of a narrow window of optimal performance. I now can see that just isn't the case.
 
#51 ·
Just a quick update for anyone that finds this thread while doing research for their own troubleshooting.

After changing all 4 O2 sensors and resetting the ECU the overall performance did improve significantly. Even with the improvements, I could still detect some minor hesitation in off idle part throttle acceleration at times. It was not always consistent and same as before I could not make it happen or make it go away when it did happen. I still wasn't getting any codes thrown so I wasn't sure where the issue was occurring. Overall everything felt like it was about 90-95% back to normal but something still wasn't quite right at times.

I decided to replace the MAF sensor with a new Denso unit from Rock Auto ($65). I had previously cleaned the MAF several times (all parts, the match head and the recessed little wires) but I was willing to accept that at 188K maybe the MAF just wasn't up to snuff any longer. I replaced the MAF and reset the computer a couple nights ago and things are now back to 100% normal. I no longer have any hesitations or flat spots in power delivery while accelerating from off idle with part throttle.

The first 1/4 tank of gas after the MAF replacement was showing signs of poorer than normal MPG but I am chalking that up to the computer settling in and me making several big stabs at the throttle while driving in effort to see how things responded. I have topped off the tank and reset the trip meter to make a new MPG calculation and so far its all looking good. I will keep the thread updated if this ends up not being the end of the issues but so far its all now back to normal and no negative signs or symptoms.

So far the biggest learning experience for me has been that the sensors can be degraded enough that its really obvious something isn't right but not degraded enough to throw a code. I had always believed that the computer would complain soon as things drifted out of a narrow window of optimal performance. I now can see that just isn't the case.
I also did the MAF cleaned it a few times then finally decided to replace it. I first went with the cheapest chinesium one I could find and it was horrible truck idled rough and rumbled on start. replaced with OEm and no problems since, Do not skimp on this part. Get OEM.
 
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#9 ·
Quick update, everything ran well and good after doing this work but a week ago I started getting nearly the same sorts of hesitation issues. This time they were a lot more consistent and happened almost all the time. I ended up replacing the fuel pump and filter with the kit from UDR and this fixed the issue this time around. I made a new thread for the fuel pump replacement here.

Knowing what I know now - I wouldn't wait for ECU codes to be thrown (which was my old stance) before replacing the sensors or fuel pump/filter because I never did get a code when things were obviously degraded.
 
#10 ·
do you think that your O2 sensors and MAF issues have anything to do with the latest on the DUSTGATE being discussed for other Toyotas (especially the 1GD-FTV). I'm being told by the mechanic that I need to replace the O2 sensors (but I've only done 88000). It keeps throwing the codes after starting a couple of times after the reset! Any suggestions?
 
#13 ·
This is a great write up. I have a '07 TRD standard transmission and the exhaust is getting soft so I am replacing with stainless/performance exhaust and wondered if you had any experience with O2 Sensors for "higher-end" exhaust systems? Does it matter enough to not use the OEM spec sensors? Any suggestions would be greatly appreciated.
 
#14 ·
I have only ever used the OEM exhaust and so far with 11 years and 210K+ miles its working just fine. Same with the sensors, OEM used Denso and thats what I replaced them with and everything seems to be working great. I think the supercharger guys might be doing some 'higher end' stuff but I dont really know. Underdog Racing would be the place to check out for the super charger stuff.
 
#15 ·
Did this today, thanks for the write-up! For me it was much easier to do the A/F sensors from the wheel wells. I kept both exhaust brackets on but removed the front propeller shaft to have better access to the driver side O2 and A/F sensors. All threads were in perfect shape.
 
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#16 ·
I changed out my 4 yesterday with the Denso's from RAuto. I did have a slight hesitation taking off but that's not why I changed them. I have a 2008 with 106K on the odometer so I figured it was time. Wow what a difference, I should have done them sooner. After replacing them it seams to idle better, stronger smoother acceleration, and the annoying hesitation is gone! It runs now like a new truck! Granted I've already replaced the fuel pump last year and all my other maintenance is up to date. I definitely used the Lisle wrench 12390 and the Lisle thread chase 12230 Romad recommended! They were both a must. My threads were good but the chase worked great cleaning up the threads so there were no issues when I cranked it up. I had most of the tools Romad used but no 1/2" extensions or wrench. It definitely would have been easier with those tools. I did the driver's side a/f from the wheel well and the top. I found a 35" steel bar I used a hammer from the top to break loose. Driver's side lower was tricky to make sure I was turning it the right way upside down. It probably would have been easier to remove the front driveshaft but I got it done with the hammer and an extension. The passenger side lower was easy. The passenger side upper a/f I did from the top. I didn't have the 22" wrench so I found a hammer with multiple size extension bars broke them all loose pretty easy requiring no disassembly of anything else on the FJ. This worked for me but maybe not everyone. Took me about 3-4 hours to do all 4. This was definitely not an easy job but I did get it done. Thanks to ROMAD for the tools list and general instruction!
 
#17 ·
I currently have just over 290,000 miles on my 2008 FJ. The Fuel/Air Sensors and O2 Sensors are still the original factory units. I've never had an issue stemming from these sensors. However, their performance has to have been degraded due to the miles they've seen. I'm guess it would be a good idea to replace these soon. It would be interesting to see if there is any change in the run quality or fuel economy after the change.
 
#19 ·
PREPARATION FOR UPSTREAM SENSORS:

Apart from regular hand tools, I found the following tools to be helpful:

Low profile Oxygen sensor socket
Set of long 1/2 inch extensions
Swivel socket holder

HINT:

Make sure the connectors are clean and not choked with mud, since you will need to disconnect them by feel. They have a little tab that must be pressed for the lock to disengage. The tab is oriented towards towards the firewall on both sides:

Automotive tire Gas Auto part Electric blue Metal
 
#21 ·
From the top, follow the sensor wire to the connector and press tab by feel to disconnect. This photo is from below to show the connector:

Automotive tire Gas Auto part Electric blue Metal


Install the sensor socket and ratchet handle from below to remove the sensor:

Automotive lighting Helmet Motor vehicle Automotive tire Auto part


The new sensor is easier threaded from the top, with the anti-seize goop on the threads, but tightened from the bottom. Take care not to foul the sensor with the goop.
 
#22 ·
DRIVER SIDE:

The access here is a little bit more difficult. The top bolts on the bracket are removed from below using a 14 mm ratcheting ring spanner, and then the bottom bolt to remove the bracket.

The sensor socket is easier to install from above, and then the ratchet handle from below to remove the sensor:

Automotive tire Gas Bottle Auto part Motor vehicle


Install the new sensor from below, using the supplied goop:

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#24 ·
Replaced all 4 sensors today ,I had 123,000 miles and running great no codes etc.I decided to take FJ tests advice and change them out,The difference is noticeable for sure ,The OX sensors were easy compared to the AF were more of a challenge I managed to do both AF from the top without removing the ex brackets and front drive shaft.
 
#29 ·
No, took the quote from Toyota to confirm part numbers and bought from Amazon
It's very hard for me to got OEM parts from the Toyota sites
Wither they block shipping to Mexico (due to policies) or shipping is high that makes it same as dealer price.

@FJtest , Should I be worried on these sensors?
Anything to confirm before installing?
They say Denso and part numbers on the sensores themselves, engraved or printed...
 
#30 ·
Where did the plastic sandwich bags with the Toyota labels come from?

Were they in the 'Denso' boxes?

If so, I'd be suspicious, as authentic Denso sensors are NOT packaged like this.

Screw-in Denso sensors will typically be shipped with a small packet of anti-seize; was anti-seize included?

Finally, are the Denso part numbers laser-etched on the sensors themselves? If so, they are almost certainly counterfeit, as Denso does NOT engrave the PN on the sensors, only an identification 'code'.

Lots of information on the web regarding counterfeit Denso exhaust sensors and spark plugs. The Chinese fakes can appear to be very similar to OEM Denso parts.

Looking at your photos, I would be suspicious that these are counterfeit parts. Contact Denso at the phone # below, send them photos, and they will authenticate your sensors.

Statement from Denso:

DENSO is offering Aftermarket (First Time Fit - FTF) Air Fuel Ratio & Oxygen Sensors to the industry. However, due to our corporate bylaws, DENSO is not allowed to sell product directly, thus DENSO has authorized distributors for the sale and warranty administration and or exchange consideration of our products. At this time EBay is not an authorized distributor for DENSO products, they are generally a selling agent. All that said:

  • Unfortunately, DENSO exhaust sensors & and spark plugs are being counterfeited.
  • As EBay is not an authorized distributor, DENSO can not control the products that are available.
  • Part Identification, is difficult, you will not find the DENSO FTF p/n on the actual sensor.
  • The DENSO FTF sensor should arrive in a sealed plastic bag inside of a box.
  • The DENSO FTF sensor # is on the box & sealed plastic bag.
  • If your sensor was not received in a sealed plastic bag, I would return it.
The identification makings on the actual sensor, can only be deciphered internally at DENSO. If you can provide DENSO the suspected FTF part number and any identifying numbers or marking's, we can try to authenticate your sensor.

DENSO Products & Service Americas, Inc.
Technical Services Department
For more questions please contact: 800-366-1123

And here's a guide from Denso showing how to identify counterfeit spark plugs, but much of the info is also applicable to exhaust sensors.

https://www.denso-technic.com/images/news/en/2020/identify-counterfeit-denso-spark-plugs.pdf
 
#32 ·
Thanks for the info and guidance...

So, the 234-4260 came in Denso box, inside there's anti seize as u mentioned and no P/N engraved. (Pic attached)

However, as u saw and commented, the 234-9026 came in this plastic bag, Toyota sticker, no anti seize and do have the P/N engraved.

Now I will start checking and confirm on these ones to see what's up

Thanks for the help
Material property Font Electronic device Audio equipment Bumper
 
#33 ·
Resurrecting an old post here…

I called my local Toyota dealership to get part numbers for the sensors. They gave me the following sheet, which I will attach. The recommendation was 3 different sensors, with 4 total. This older post has 2 sensors. And I will attach the corresponding part numbers from the rock auto website. Anybody have any feedback, I would appreciate it.
Font Material property Paper Parallel Document

World Map Font Parallel Technology
 
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