Repeating O2 sensor codes :(

Hey man encountering this same exact issue right down to every detail. My mechanic installed Denso OEM down stream sensors and CEL came back on 15 miles down the road. We tested everything from the fuse box down to the sensor with no luck of why. I took it in to Toyota for a diagnosis and all they said was replace o2 sensors with OEM... which were replaced with OEM parts (Denso is a Toyota manufacturer) Toyota takes Denso and repackages them into Toyota stamped boxes. Mechanic informed me Toyota has me by the balls and wants me to buy their products so they can "flip" off the issue by spending money. Dealer has parts on order and I'm biting the bullet on Monday.

 

However if it does flip back on I'm coming right back for a full refund on parts and diagnosis for being incorrect... than I think I'll try it your way and replace both AF sensors and both down stream o2s with Wholesale Denso parts from my mechanic.

 

Have you checked your your heater a/f relay and your Ecu relay via the fog light socket?

I had the same issue for four years and kept pulling my hair out. Spent probably over $1k on upstream O2 sensors, had https moduleexperts.com/ rebuild my ECU, replaced the following:

1. Upper intake gaskets
2. Lower intake gasket
3. Intake plenum (gasket)
4. Water pump
5. All serpentine belt pullys
6. Serpentine belt tensioner
7. Spark plugs
8. Serpentine belt

And finally began to gag on exhaust in the cab.

Crawled under the truck to search for visible gaps that might be exhaust leaks. Sure enough, there was a 1/4" - 1/8" gap the entire length the bottom of both headers.

Replaced the exhaust manifold studs with OEM from the dealer, and used high temp lock washers under the new OEM nuts. Replaced the header gaskets and boom, no more CEL!

The exhaust smell in the cab is what finally gave the culprit exhaust leaks away.

 

Well me and my mechanic checked everything electrical and that's all fine. I haven't found any exhaust leaks but I could be wrong... atleast haven't smelled any. As for the actual A/F sensors im gonna let Toyota do their thing and if it still pops on I'm gonna raise hell for a refund than do it my way by replacing the A/F sensors along with the o2 sensors and go from there.

 

It took four years after I installed the Doug Thorley long tube headers for the smell to reach the cab.

The issue, I am thinking, was that I failed to use lock washers on the exhaust manifold studs.

Apparently a professional is able to check for exhaust leaks beyond my level. I wish I would have had it looked into professionally early on as an exhaust leak issue.

I don't think the repair shop checking for exhaust leaks is super spendy at any rate.

 

Definitely gonna have the exhaust looked at before this escalates to a lot of money. Thanks for the help

 

No worries. If the leak is there it is between the engine block and the first or upstream O2/AF sensors.

My trucks CEL vexed me for seriously four (4) years. I would replace the upstream O2/AF sensor reflected by the code, the CEL would go away, then two or three weeks later, there it was again.

For a long time because the general consensus appears to be that O2 sensors do not go bad I assumed mine were becoming dirty, so to combat I would remove, soak in gasoline overnight, and reinstall all to no avail.

I attempted every home vacuum leak detection method possible, for instance spraying brake parts cleaner around the engine while it was running and at no time suspecting it was an exhaust leak.

Actually, I do remember coming up with an exhaust leak very quickly into the CEL and then dismissing it as a cause after I had the exhaust shop weld my performance Y pipe and muffler. I could not conceive that my fancy long tube headers were the culprit. The same muffler shop now has my old header gaskets on display in their shop, the leaky ones, and they appear to be only the outline of gaskets with interlaced wires all that is being left.

I suppose the exhaust did not begin to make it into the cab causing the gagging until the gaskets completely wore through.

If you do find a leaking exhaust manifold or header gasket I would replace/have it replaced along with all the studs and hardware to include lock washers. This is because when I finally noticed the gap I of course immediately decided to fix by cranking down on the exhaust manifold nuts. I did this regularly afterwards because the CEL continued to come on until, one day, the front right header nut kept on turning without any resistance and then it came off in my wrench.

All the bolts in my '05 engine are rotted for some reason. I hate to buy all new hardware to replace something in there, however if I do not I invariably break a stud or bolt - and that is even without using air tools. Here is what came off:

 

Is it pricey to replace the gaskets and all hardware? Also is it a pain to replace the studs? I'm pretty handy with toyotas but sometimes I prefer a shop to do certain projects.

 

UPDATE well had the exhaust smoke tested and I was right in there with my mechanic. No manifold leaks.... I'll be experimenting with the o2s and A/F sensors now and hopefully I can get to the bottom of this nonsense, last resort after those is messing with the cat

 

Take a look at the connectors at the ecm for atf. I work for at a toyota dealer and have seen a few of these, recurring O2 sensor codes and finding atf at the ecm. Fluid leaks from the harness in the trans and wiks up the harness into the ecm. If so: new ecm, engine harness, internal transmission harness. Big ass job, you have to replace all of that.

 

I have a manual 6 speed, didn't find any leaks while I was under there.... been at the dealer for about a week now and they can't seem to find out what the issue is, they did o2 sensors and A/F sensors still no change, exhaust was checked before I dropped it off as well. Everyone seems to be scratching their heads.

 

Any update on this?

 

Caveat: beyond knowing your fuel trims need to be within 1.0 or better of each other I cannot tell you why or even *what* a fuel trim is?

My best theory is that since my JBA long tube headers were not easily attached to my Y Pipe they are “yearning to be free” and require lock washers on the header studs.

Since posting above I had my engine replaced and sure enough, after driving seven hours at highway speeds I watched my fuel trims go from 1.0 apart to 20 apart.

That is when I noticed the mechanic failed to transfer my lock washers to the new manifold studs.

I was planning a bigger write up and do not have pics yet. however I purchased the Dezod stainless ARP studs and after the eternity it took due to Dezods wedding planner for the studs arrived I began the installation.

Let me point out the ARP exhaust manifold studs are far easier to install if you remove the headers.

I did not chase all the threads with a tap and did not have any significant issue installing, however you are better advised if you do.

I used anti seize on all the threads and faced the ARP washers bevel side out towards the nut.

The best nut tightening was accomplished with a 1/4” 10mm deep multi point socket. I ended up taking the wheels off for a separate project which allowed me to access all the studs except the far front one on the drivers side.

All was good until a month later when my wife reported the CEL was back on. So I checked the header nuts and not only was every single nut loose but several/most had some sort of tan “seepage” barely visible on the ARP stainless threads just outside the nut.

I reset the CEL and went on a seven hour deer hunting road trip.

I can watch my OBD II settings on my iPad with this dongle and dashcommand. Approximately six hours into the trip my previously close fuel trims changed and the right side went to 4.9 and stuck there.

Sure enough, my cousin in Kansas had a 3/4” 10mm deep multi point socket and the ARP nuts I could reach were loose. After that, my fuel trims went back to close again.

I have been on vacation from after hunting until now and thus have no pics or notes to add.

I have ordered these The Hillman Group 3640 M10 Metric Spring Wave Washer, 25-Pack

Hillman 3640 M10 Metric Spring Wave Washer, 25-Pack

https://www.amazon.com/dp/B00I2VV4TU?ref=yo_pop_ma_swf and plan to install at first opportunity.

I had the M8 version of the above and they almost fit. The M10’s look a little big but I plan to install regardless. Perhaps the M9’s are correct?

Will post pics when I have them.

 

Do you have enough stud sticking out to double nut?

 

Is that better than a lock washer?

Not completely, however the ARP studs are longer than factory. I will measure both as I have one left over.

 

I had a cel for a O2 sensor. Turned out to be the donut gasket where the manifold connects to the Y pipe.

 

Good point.

About two (2) years into my CEL I took it to the muffler shop across from Lackland AFB. The muffler shop checks and repaired all exhaust leaks.

They either failed to identify the leak across the bottom of each header or it was not at the 1/4” to 1/8” gap size when I finally discovered.

They do have the remainder of my gaskets on display at their cash register because the only thing left of the gasket was a latticework of wire. Must be the worst case they have seen?

 

Where'd you get the studs? Link?

 

The lock washers you see in the above pics resolved the issue on the original engine. Sure enough, when the shop replaced the engine this summer and did not transfer the home depot lock washers the CEL came back!

I got the pretty ARP studs from Dezod - referenced on here, found via google. The home depot lock washers on new factory studs resolved the issue, however they are not nearly as pretty as the stainless.

These sockets work to tighten the shiny stud nuts:

Zenith Industries ZN502233 1/4" Drive 12-Point Deep, 4mm - 14mm Metric Socket Set

https://www.amazon.com/gp/product/B01K4MIEUG/ref=oh_aui_detailpage_o03_s02?ie=UTF8&psc=1

These female torx heads remove the factory studs:

Anytime Tools 11 pc Female Star Socket Set with Rail E4 - E20

https://www.amazon.com/gp/product/B000I45Y8Q/ref=oh_aui_search_detailpage?ie=UTF8&psc=1