O2 Sensor Curiosity Question

[sraney]

My 2001 Tundra finally generated its first error code a couple of weeks ago at 116,000 miles--P0155 O2 sensor. Using this forum as a guide, I dutifully replaced the Bank 2 Sensor 1 unit. The truck then promptly (within 12 miles) generated a code (P0135) for the same sensor, opposite side (Bank 1 Sensor 1). My search of the O2 threads over the years shows that this is a pretty common occurrence--a code for one side followed closely by the corresponding code for the opposite side.

I'm not really much of a mechanic, and I wonder why this pattern occurs (as I prepare to order yet another $100 OEM sensor). While I would expect a similar MTBF for each side, I can't believe that the sensors fail independently so close together. Can the computer only display one code at a time? Was the other code just "waiting" to appear once I fixed the first one? Can the computer only diagnose one sensor failure at a time?

I welcome any comments from you experts out there to satisfy my curiosity.

Thanks.

[duffyatkinson]

I think what happens is that the ECU finds fault on a comparative basis, i.e. the old sensor, while still within parameters, is registering values so far away from the new one that the ECU determines this to be a fault all on its own.

Also note that the O2 sensors on our Tundras are the 4-wire type, wherein 2 wires are used to bring the unit up to operating temperature and the other 2 are the data communications circuit. It's possible that a differential in operating temperature(s) vs. time can throw a code as well.

Don't be surprised to find that you will need to replace all four to keep CEL codes from popping up. And stick with quality Denso or NGK sensors. If you are doing your own R/Rs, hit the base on the front ones and the nuts on the rears with a good blast of WD-40 or PB Blaster beforehand and let it sit for an hour or so to help with removal. Make sure to replace the flange gasket on the rears.

[sraney]

Thanks for the prompt reply. That makes sense to me--the difference in both heater and sensing performance between the two sides is probably pretty large at the moment (the P0135 is technically a heater code, I believe). That would explain why this "sequential" pattern of failure codes appears to be fairly common. Oh well, it's only money. Thanks again.

[sraney]

P.S. Yes, I'm using Denso OEM sensors, and I second the PB Blaster suggestion--I used it liberally and let it soak for an hour, and it took only moderate pressure to break the sensor loose. The pipe threads remained pristine, thank goodness!

[sraney]

For those few of you who have been following the "Zen of Oxygen (sensors)" thread, here's the final installment, at least for a while. Led by Zen Master Duffy, I've been exploring what kind of O2 comparator circuitry is built into the ECU, i.e., why does replacement of one sensor seem to almost immediately trigger a code from the other sensor? Does the ECU only compare voltages between the front ones (and maybe between the rear ones), or does it also have some kind of comparison running between front and rear ones as well?

Based upon my empirical research, here are my conclusions (so far). The ECU compares voltage between the right and left front sensors (i.e., between bank 1 sensor 1 and bank 2 sensor 1). This means that if you replace one of the front ones after it's had a few miles (in my case 116,000 miles), you can expect a code to be generated from the remaining "old" sensor within just a few miles simply because of the different response time/voltage/whatever being generated by the two now disparate sensors. However, after replacing the remaining front sensor, I've now logged approximately 100 miles without seeing any codes from the old rear sensors. (Of course, now that I've posted this, I'll probably see a code tomorrow!) Thus, I tentatively conclude that the ECU doesn't care that the front sensors are new and the rear sensors are old. With a little luck, I'll make a few more miles before the rear sensors hit their limits and start throwing codes.

Here's my second conclusion: Replacing these sensors can be really easy, or it can be really, really hard. After soaking in PB Blaster, the first one popped loose with a six-inch ratchet with no trouble. After soaking in PB Blaster, the second one (bank 1--driver's side) resisting cheaters, torch heat, manifold heat, begging, and cursing. It finally took an impact wrench, which, of course, caused minor damage to the threads in the header. However, running a chaser through cleaned it up satisfactorily.

Thanks again to Z.M. Duffy for his previous comments regarding the zen of O2.

Grasshopper

[derone]

I think the O2 sensors are actually still working with those codes. It's the heater element within them that goes bad. So if the heaters reach light-off temperature at different rates, then yea, you'll see a code for the other front O2 sensor after replacing the first one. The rear sensors rarely go bad.

[rotarex]

this is the reason i spend $375 usd and ordered all oxygen sensors, Eff it, change em all, i dont got time to fiddle with it week after week, i ordered it on monday hopefully its here in canada by next monday im getting 550 kms per tank when i change it i should get close to 750 kms, i was getting 675 kms before it went and still had 10-15 liters in the tank so close to 750kms with about 2L in take is achieveable thats 17.5 MPG average

[chinaloy]

i changed my front sensors a year and a half ago, approx 9000 miles. I have just changed my bank 2 sensor 2. I have not changed the other one yet, because the weather stinks.
i do not think there is a correlation between the fronts and the rears.

BTW, i have about 135,000 miles on the truck.

I used oem sensors for the front and am using Bosch universal sensors in the rear.

so far so good

[duffyatkinson]

Originally Posted by chinaloy

i changed my front sensors a year and a half ago, approx 9000 miles. I have just changed my bank 2 sensor 2. I have not changed the other one yet, because the weather stinks.
i do not think there is a correlation between the fronts and the rears.

BTW, i have about 135,000 miles on the truck.

I used oem sensors for the front and am using Bosch universal sensors in the rear.

so far so good

There is a SIGNIFICANT relationship between the upstream and downstream sensor units. The upstream (air/fuel mixture) sensor gives direct feedback to the ECU to enable an efficient fuel/air mixture given engine circumstances. The downstream (emission oxygen) sensor helps the ECU monitor the effectiveness of the catalysts system's emission control functions. The ECU has to monitor not only the sensor readings but also the heating up ("lighting off") cycles of the sensors and catalyst components as well, i.e. the ECU ignores the downstream readings until the cats reach effective operating temp. (The sensors receive voltage to get them up to operating temp quickly, just a few seconds). The ECU uses the data returns from BOTH packages to make sure that the fuel/air mixture stays around the stoichiometry (for gasoline - 14.7:1), operating the catalyst most effectively.

And dump the Bosch components. They'll cause you problems down the road. Stick with NTK or Denso. The Toyota ECU is really picky about you trying to use "universal" components here...

[chinaloy]

Duffy, what i meant to say was that just because the front sensors fail, it does not mean the rear sensors will follow in so much as if one front fails, it seems that the other front follows very shortly after.

As for the Bosch vs. NGk or Denso. I have not seen empirical evidence saying that Bosch is bad for Toyotas.

As a matter of fact, I just replaced my spark plugs with the oem Densos and my mileage went down and my truck cranks over more before it starts. My orig plugs had 135k miles on them and the gap was huge, but the truck ran better. go figure.

It's electronics, the ECU doesn't care who made them, it just reacts to them.

I have come to the fact that O2 sensors are wear items and they need to be changed periodically. I used the universals because of cost. They are easy enough to change. If I need to change them out 5000 miles earlier, then so be it, they were half the price of oems.

[duffyatkinson]

Originally Posted by chinaloy

Duffy, what i meant to say was that just because the front sensors fail, it does not mean the rear sensors will follow in so much as if one front fails, it seems that the other front follows very shortly after.

I agree. But as you mentioned below, O2 sensors ARE wear items, so units installed at the same time can be expected to fail at the same time.

Originally Posted by chinaloy

As for the Bosch vs. NGk or Denso. I have not seen empirical evidence saying that Bosch is bad for Toyotas.

Didn't say bad; I said that significant anecdotal evidence indicates that the ECU in the Tundra does NOT like to work with the Bosch units. I'm not an engineer, so I can't tell you why. You decide whether you want to gamble on a German component in your Japanese vehicle.

Originally Posted by chinaloy

As a matter of fact, I just replaced my spark plugs with the oem Densos and my mileage went down and my truck cranks over more before it starts. My orig plugs had 135k miles on them and the gap was huge, but the truck ran better. go figure.

If all you did was swap plugs, then something is distinctly wrong. You either have used an inappropriate plug, or your gaps are off.

Originally Posted by chinaloy

It's electronics, the ECU doesn't care who made them, it just reacts to them.

You are correct. It IS electronics. And they rely on a certain set of expected behaviors that correlate to its operating rule set. When there is deviation (as may be expected with non-OEM electronics), then unsurprisingly there will be operating problems.

Originally Posted by chinaloy

I have come to the fact that O2 sensors are wear items and they need to be changed periodically. I used the universals because of cost. They are easy enough to change. If I need to change them out 5000 miles earlier, then so be it, they were half the price of oems.

You are correct. They have electro-ceramic components that wear during use. But the OEM Denso and/or NTK units have an expected operating life of 100,000 miles in normal service.

[Ando140]

P0135 & P0155 are both heater circuit malfunctions, same thing happened to me. I had one go out, replaced it and the other one immediately went out.
I don't think the ECU compares the two sensors with regard to the heater circuit. I found in my service manuals that you can actually check the heater circuit with an ohmmeter. I can't remember what the resistance is supposed to be, but when I checked the old sensors I found the heater circuit was open and the new sensors were spot on the expected readings.
So, I think what happens is the heating element in the sensors actually breaks, thus opening the circuit. The fact that they tend to fail at the same time, I think, is a testament to the consistency of the quality (good or bad) of the sensors.

[Tundrav8yamaha]

Originally Posted by duffyatkinson

I agree. But as you mentioned below, O2 sensors ARE wear items, so units installed at the same time can be expected to fail at the same time.


Didn't say bad; I said that significant anecdotal evidence indicates that the ECU in the Tundra does NOT like to work with the Bosch units. I'm not an engineer, so I can't tell you why. You decide whether you want to gamble on a German component in your Japanese vehicle.


If all you did was swap plugs, then something is distinctly wrong. You either have used an inappropriate plug, or your gaps are off.


You are correct. It IS electronics. And they rely on a certain set of expected behaviors that correlate to its operating rule set. When there is deviation (as may be expected with non-OEM electronics), then unsurprisingly there will be operating problems.


You are correct. They have electro-ceramic components that wear during use. But the OEM Denso and/or NTK units have an expected operating life of 100,000 miles in normal service.

nice sig see you received my PM

[duffyatkinson]

Originally Posted by Tundrav8yamaha

nice sig see you received my PM

It's a great addition. Thanks for getting me set up! I'd like to see all of us have it; it's a quick and easy way to see where all of us are writing in from!

Duff

[single]

I don't like your sig! It's 16 degrees here in Oklahoma.