I have a 99 limited with 129K miles. The first time this happened was about six months ago. The check engine light came on and I don't have a scanner so I took it by a local repair shop and they said it was just a dirty 02 sensor. They took them out and cleaned them. Problem solved...I thought. Check engine light came on again a few days ago so I took it to the same repair shop and they scanned it again and cleaned the 02 sensors. This time it took care of it for 2 days and the light has come on again. They told me at the shop that it would cost $250 or more to replace the 02 sensors. My question is...is this a DIY job or should I let the shop take care of it? Honestly I don't know the location of the sensors (I believe there are three??) and the difficulty in replacing them. Does anyone have a writeup on replacing them?

 

Because the shop has recently removed them, you should have no problem replacing them. However, you should make sure you need to replace both. The problem may only be with one of them. The ODBII scan tool can tell you that. Most likely it is the front sensor.

They are normally just in front of and just behind the catalytic converter. Which is probably right under the passenger side floor board.

The hardest part might be figuring out how to get the wire connector apart. It could help to get the new sensor(s) first so you can see how the connectors work.

You can get a scan tool for under $100 dollars if you have a portable (Windows based) computer. Probably the scan tool will cost about the same as one of the O2 sensors. So, if you get a scanner and it saves you the cost of one O2 sensor, that is not a bad deal. I got this one: ODB II to USB. It worked well for me.

I read a number of posts that indicate the Toyota ECU does not like aftermarket O2 sensors. It is tempting to save money by getting aftermarket but be aware that might cause you problems.

jrohland

 

take it by autozone and they will check it for free and then u can know and wont have to drop the $100 on the scanner yourself

 

Thank you for the info. So there are only two sensors? I will run by Advance Auto and have them scan if for me. Are the sensors screw in or held in by some type of fastener? Any high temp silicon required when replacing or any special tools? Sorry for all the ??'s Just want to be prepared when I go to replace one or both.

 

x2 on autozone or advanced auto. They will check it for free and tell you whether it is the front o2 (bank 1 sensor 1) or Back (bank 1 sensor 2) . You should only have 2 sensors. If you can turn a wrench you can tackle this yourself.

http://www.lieblweb.com/tacoma/Mecha...placement.html

Heres a write up for a tacoma. Should be very similar in your 4runner. I've just replaced both of mine within the past few weeks. All you have to do is undo the nuts that hold the flange in the exhaust pipe. Pull the sensor out. put the new one in and tighten it back down. The hard part is the wire connector for the front sensor. In mine i had to go through the shifter boot and it was TIGHT. An extra set of hands can be helpful. (one person in the cab and one underneath the truck) Definatly do NOT get aftermarket sensors. Go to your dealer and get Denso replacements. The rear sensor was a piece of cake. Follow the wire from the sensor up through the grommet and the clip is under the carpet on the passanger side floor. (inside the cab.)

FYI be extra careful when removing the nuts that you do not twist off a stud. If it starts to turn really hard. STOP spray it with a little bit of penetrating catalyst like PB blaster and wait a little. I twisted a stud off last week and it was a pain in the butt to get the old stud out and put a new one in. Just take your time and follow the write up above and you should be fine. the nuts that hold your o2 sensor in should be 12mm. Also make sure you get a new gasket and new nuts when you pick up you new sensor. Not a bad idea to put some antisieze on the threads while your at it. You said that the shop has already taken them out twice now so hopefully they will come out easy.

 

Changed my sensor last week and it took about 10 minutes. Simple and easy fix.

 

I will give it a shot today or over the weekend and post back. Thanks for all the info.

 

Just went by the Advance Auto and had them scan it. Came back as trouble code P0420 and the scanner stated "catalyst efficiency below threshold bank 1"
Would that be the 02 sensor or the catalytic converter causing the problem? Gentleman at Advance Auto was no help...

 

https://www.yotatech.com/forums/f2/p...e-tech-145496/

This should point you in the right direction.

Heres how to check your O2 sensors.

http://www.ncttora.com/fsm/2003/Repa.../afrs/insp.pdf

Heres a link to the Factory service manual for future reference. There is a crapload of good info in there.

http://www.ncttora.com/fsm/2003/index.html


Are we having fun yet????

 

If you can i would try to check your 02 sensors out before you go throwing expensive parts at this problem. o2 sensors for the rear are about 90$. the front one is about 120$$. and a new Cat is.....well you dont even want to know. So...the good news is your fine to drive this around for a while while you get this figured out. You might not get the best MPGs but your not going to detonate your truck or anything. If it takes a week to figure it out but you get it right the first time you will be much better off than just throwing good money away.

Keep us posted.

 

just do both. they die about the same time. 100,000 to 150,000 miles give or take.

replace one now, I guarantee a code on the other one in 4 months...

they get old and can't do the job anymore it has nothing to do with them
getting dirty...the sensing elements themselves lose material over time

 

This is true.

 

Make sure you get a gasket w/ the new o2 sensor(s) if you end up replacing them - the 1st sensor I bought didn't come w/ anything. I also had to spray a little wd-40 on the bolts and let it set for 10 minutes before they would loosen w/o stripping. Don't force them!

Here's another write-up which also covers the easiest way to unplug the wiring:

http://www.4x4wire.com/toyota/4Runner/tech/O2_Sensor/

 

Thank you. That article was exactly what I was hoping to find. A previous poster had indicated you had to go through the shift boot, but I saw the connector today and thought you had to be able to replace these without that much trouble. Thanks again.

 

Glad to help. Good luck!

 

My thoughts:
If you have never changed your sensors at 129K then they are both due. I got 75K off my front (Cali. A/F type) and 90K off my rear. Buy Denso, and only Denso from dealer, URD or Amazon (good prices). You just need to be SURE you have the correct Denso part number; the URD website is a good place to get those. If a seller does not ask you auto or manual, Cali or 49State, and engine, then you stand a chance of getting the wrong unit.

Buy new nuts at the dealer, if they are not already shot, they will be by the time you have to change it the next time.

I don't know why you would HAVE to go through the shift boot on a 4Runner; I have no idea about Tacos. You COULD do it that way on the 4Runner Cali set-up; but it's easier to unbolt the sensor first and then pass it up and over the transmission. Then it all hangs down nice and loose and you can undo the connector last. None of the write-ups I have seen describe this trick for the Cali but it's not hard to figure out.

I did both of mine and each took about 20 minutes thanks to rusted nuts. Cured my P1135 (front) and P0420 (rear) codes. It is my belief that you can test a sensor and know if it is bad, but it can test ok and still be degraded enough to throw a code. So if it is over 75k on the clock AND you have a code, I would change whichever one is called for by the DTC code. Fronts usually go first, so if both are original--that would be my bet. If after changing the front, don't be surprised to see P0420 soon after, then it's time for the rear.

 

check engine light has not come back on and the 02 sensors had to be put on hold because I found a more immediate problem. Turns out I also had a broken end link on my front sway bar. Had both replaced over the weekend. Will post back once I have had a chance to replace the 02 sensors.

 

Does anyone know where to buy an A/F ratio sensor (plus gaskets) online for a reasonable or cheapest available price? I have the Cali type design, so an oxygen sensor will probably not work for me.

Thanks
OO

 

URD sells the OEM Denso sensors for a good price and has reliable, detailed application info. You need to know year, engine, Cali/49, manual/auto, and front or rear sensor.

URD Linky

You can possibly get a better price on Amazon, but get the Denso part number from the URD site first to be safe. Amazon's application info is ambiguous , if not sketchy. Then go buy nuts from the dealer, odds are you will need them. Denso sensors come with the gaskets but not the nuts.