Running Rich, Bad O2 Sensors?

Hi everybody, long time lurker of the forum and just joined. My 2006 Sequoia w/ 178,000 miles just threw a CEL and codes p0138 (B1, S2) and p0158 (B2, S2.) I had originally thought it was only p0138 until I double checked and now I seen the p0158 and so I have the downstream and upstream sensors for that first side. Now, my mechanic friend took a quick look today and noticed it running rich and says doing the o2 sensors might be a waste of time and it could be something else making it throw the high voltage code. But from what I’ve read on here, bad o2 sensors can cause it to run rich?I also went ahead and just cleaned the MAF sensor which didn’t even look too dirty. What do you guys think? Should I just get the other 2 sensors and do all 4 or would that be throwing parts at it? Or could it be something else? Thank you.

 

It will run rich at startup (until the O2 sensors heat up), and if the O2 sensors are not operating properly. Follow the codes.......

 

Look, it could be a number of things. Bad sensor at either end of the cat can red-herring you into thinking the 'good' sensor is bad. It's not unheard of to see both sides barking on one sensor also. We recently had a guy who was throwing P0137 and P0158 together, both rears. The advice in that thread along with this list of codes from the FSM with possible sources may shed some light.

Step back for a second and think about this logically. Your O2 sensors are responsible for (1) feeding your ECU info on whether you're running rich or lean as it checks combustion coming out of the exhaust pre-catalytic converter so it knows if the engine's air/fuel mixture needs adjustment, and (2) what that exhaust gas mixture looks like after it passes through the catalytic converter, so it knows if the cat is working correctly. ECU is also using that data to decide on timing, and of course your engine needs a solid strong spark from a legitimate plug (not some legit-looking knockoff you bought cheaply on scAmazon or fleaBay) to fully ignite the mixture optimally, with minimal air/fuel waste.

Think about it though - a primary function of your B1,S1 and B2,S1 sensor are trying to determine whether the combusted fuel/ox ratio is good based on what it's detecting in the exhaust. The ECU is using the MAF to determine airflow, and the throttle body is also regulating that (automatically on your '06). What three things are constant here, to produce that mixture your upstream O2 sensor is checking?

• Metered air
• Strong, well-timed spark
• Metered fuel

You can see from the Toyota FSM chart image linked above - obviously both codes boxed in red aren't yours, 1st one isn't at least - but you can see it's offering up pretty simple sources with both codes:

1. Short in circuit
2. ECM internal circuit malfunction

This is what I hate about OBDII codes. That's a pretty limited window if you take it at face value. Hell, it's a limited window if you take any OBDII code at face value.

On either of those two options, if someone stole your cats and cut your wiring at some point and you failed to mention it, or some fuckwad used non-Denso sensors that required cutting the factory harness, it could cause either circuit malfunction.

If the ground line for B1 + B2 harness is corroded - like you live in a shit state where they salt the roads and failed to mention it - sort of like you failed to mention which engine you have (I assume a V8, but the V6 in '05-'06 had two banks also!) - but whatever, corrosion of the ground for that circuit could be a culprit. This could be something as stupid as checking your grounds, unplugging and replugging the O2 harness connectors a few times to get a clean circuit, and resetting the codes.

If it were me, I'd be testing the circuit for the O2 harnesses to confirm voltage and ground is adequate. But you also need to be realistic here ... at 178k miles, I'd also feel like, "Damn, I had a pretty good run these past 15 years (assuming you've never swapped a sensor), and maybe I should replace all 4 sensors with new Denso sensors!", hell, I had my 1st sensor go bad a couple years ago at 66,566 miles. It's generally frowned upon to replace just one, always replace both upstream/downstream on the reporting bank.

But you should also know, cracks in your exhaust manifold, a loose or snapped manifold stud, or similar could allow the exhaust stream to be tainted with other gases prior to hitting your front O2 sensor also.

If your mechanic doesn't work often on the 2UZ, take his advice with a grain of salt. It's a very well-known engine but it has its quirks. It truly seems like we see a lot of red-herring cases where the ECM just gets which circuit is wrong. We *don't* often see where, for example, 2 rears go out at the same time.

 

PS - it would be interesting to know the spark plug change history on the truck. Aside from confirming the engine. And aside from confirming where regionally you live. And if O2 sensors have ever been changed. And if you've had any work done where the exhaust or intake manifolds came off, or the cats were off, any exhaust work. All of this is important info.

 

Yes, my Sequoia is a V8 (thought they didn’t come with V6s,) and I live in SoCal and the truck has been here it’s entire life. I should clarify that I’m the 2nd owner and bought this truck a few months ago with no issues whatsoever when I got it up until this point. Maintenance history on it looks good (oil changes, brakes, etc. ) but seeing as his shop uses some aftermarket parts for those things, I can only assume they probably messed up on the o2 sensors and/or the spark plugs (if they were ever changed.) It’s also worth mentioning that somebody had attempted to steal PO’s catalytic converter but failed, and in that processed probably screwed up the wiring harness.

 

Also, there is really no repair history on the intake manifold/exhaust manifolds. Like I said just normal maintenance stuff and timing belt changes. I will probably take a peek at the spark plugs and see what those look like as well.

 

From your advice, I will check out the wiring harness and circuit and see if that’s all good. I will also purchase the other two o2 sensors and just get them out of the way to eliminate a possible culprit or potential future repair. I will check it out toward the end of the week, my schedule is really busy right now. But I’ll keep you guys updated

 

I think you're right on Sequoia, my miss. And yup, based on the added info, that's what I'd tell you...

• Easiest stuff first: Go to each sensor. Trace the line from its exit on the sensor to the harness where it plugs in. Look for any splices, burns, etc. Look at the wires; do they look fresh and new, or old (you can usually see some exposed blue/white/black wires), and also, does the plastic connector on the O2 sensor match the age/oxidization of the plastic harness it's plugging into? If any splices, nicks, etc. are visible, it may be your issue. O2 sensor circuit in some vehicles is susceptible to fluctuations, circuit breach, repairs. If there's a clear age diff between the plastic connections, i.e. the O2 sensor is newer, maybe the PO had issues, and swapped that sensor - if so, which sensors are like that? If not, maybe you're on original sensors, and should be cool w/ponying up the $$$.
• As stupid as it sounds, check the spark plugs since you have no known history. I'd specifically pull 1, 2, 7, 8 and give an inspection. Yes, I would expect to see misfire codes if it was the plugs. But give yourself a maintenance baseline. Consider new NGK, specifically the p/n your Owner's manual says. It's one place I opt for non-Denso. And buy from a local/established source if you replace.

One thing you may do, rather than doing what you said (I think you're saying this), i.e. buying two new rear sensors, consider replacing the upstream and downstream on one side specifically. I only say this because we've seen one or two guys come on here with your 2UZ throwing codes for a bad downstream sensor, but ... it ended up being the upstream feeding bogus info to the ECU, which interpreted the downstream as the problem.

And if you need to see the 'connector age' issue, check this. You see the clear difference in plastic age here? The sensor (off to the left) was clearly replaced recently. Wiring is virtually brand new. Before you buy anything, check to see if there's any obvious sign something was replaced. It may just be that the PO replaced them, but bought parts from a known counterfeit source like scAmazon or fleaBay, and you have a bogus grey market/knockoff part. It would be a huge factor in what path I went on this. And if they installed a non-Denso part, O2 sensors are one of those "big 10" parts you don't want to go aftermarket on. Fuel pump, coil packs, starter are a couple of others.

 

Here are the photos of the bank 2, sensor 2 as well as the wiring harness to it. Note: the sensor looks old, probably original

 

Here is Bank 1 Sensor 2 where they cut the cat. Cannot tell if sensor is Denso or not, I tried looking for writing on it. Wiring harness looks to be ok, correct me if I’m wrong though.

 

New denso downstream sensor for reference

 

Based on pics, B2S2 definitely looks well-used to me, for sure. B1S2 looks relatively new. They look inserted fully to me, they can be a pain in the ass to separate to check. Everything from the harness to the sensor looks fine, and I'm not seeing anything up-harness that looks jacked either.

Correct part for your downstream is

$29 Denso 234-4260 Oxygen Sensor

Denso 234-4260 ($51 from Denso directly), upstream is

$130 Denso 234-9051 Right-Bank Upstream with 4-Wire 8.5” Option Air Fuel Ratio Oxygen Sensor

Denso 234-9051 ($144 from Denso directly). They make it really hard to read the "DENSO" branding, almost like it's acid-etched on. Like this, below.

One of the common signs you've gotten a knockoff if the body is the same is the font size, spacing, text layout, etc. Manufacturers do change that from time to time, so not definitive. I assume you bought from a trusted source.

Anyway, that aside, you know, two things. One, you should check the fronts. It's entirely possible even though B1S2 is throwing the code, B1S1 is original and actually failed. It may very well be B2S1 and B2S2 are original. Two, something you can do (poor man's diagnostic) is swap B1S1 and B2S1, clear your codes, and see if the codes pop the same. If you pop different codes for different banks, I'd be suspicious of the front sensors. BE SURE TO PULL THE NEG BATT CABLE while doing the work for at least 5-10 minutes. It'll force the ECU to reset and re-learn. Truck may run like ass the first couple dozen miles while it re-learns; it's normal.

Hopefully this all makes sense, meds doc has me on are making me a little loopy.

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PS - RockAuto is cheaper on pricing and sells guaranteed legit parts.

Downstream is $43: https://www.rockauto.com/en/moreinfo.php?pk=1606676&jsn=726
Upstream is $115: https://www.rockauto.com/en/moreinfo.php?pk=1606992&jsn=734

Hold on and I'll edit this with a 5% off coupon code.

Rock Auto Coupon Code: 238503243177619741

Good until August 18, 2023

Using your discount code is easy! Enter your discount code in the 'How Did You Hear About Us?' box without any other numbers or characters for an additional 5% savings. Discounts cannot be applied to shipping, gift certificates, or to an order once it is marked as shipped.
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Wish I had that when I ordered the other 2 o2 sensors lmao. Yes I got them from Rockauto and they are Densos. I made to sure to do my homework on these parts and read through the forums here.

 

Do you think I got a knockoff based on the picture I posted of the new o2 sensor? I got it from Rockauto…

 

Nope. That's about what my sensor looked like when I had to swap mine out 2 years ago.

The one I snapped up above is, no doubt, several years old.

 

So a bit of an update. Sorry about the wait, been very busy lately. Anyways, last week, I went ahead and replaced the 4 sensors, cleared the CEL, and so far after driving for a week, no more CEL, knock on wood. Yay! I know how much y’all hate when people don’t follow up with issues their truck is having (I do too) and I didn’t want to be another “one of those guys.” Found the 2 on driver side were some aftermarket junk, and the two on passenger looked factory. Thank you guys for the information.

 

Fingers crossed those thieves didn't arc or short something. Also another potential testament to OEM vs. aftermarket.